Continuing on the main highway, there's little between Cataviña and the 28th parallel, where an enormous metal monument and a hotel mark the border of the states of Baja California and Baja California Sur; you'll have to set your watch forward an hour when you cross, unless Baja California is on Daylight Saving Time (April– Oct), in which case there's no change. GUERRERO NEGRO, just across the border, offers little in the way of respite from the heat and aridity that has gone before (winters, however, can find the town quite chilly). Flat and fly-blown, it's an important centre for salt production, surrounded by vast saltpans and stark storage warehouses. At most times of year you'll want to do little more than grab a drink and carry straight on. In January and February (and, peripherally, Dec & March– May), however, Guerrero Negro is home to one of Mexico's most extraordinary natural phenomena, the congregation of scores of grey whales just off the coast.

The whales, which spend most of their lives in the icy Bering Sea around Alaska, can be watched (at remarkably close quarters; the young are sometimes left stranded on the beaches) from an area within the Parque Natural de la Ballena Gris, which surrounds the Laguna Ojo de Liebre. The laguna is also known as Scammon's Lagoon after whaling captain Charles Melville Scammon, who first brought the huge potential of the bay to the attention of rapacious whalers in 1857. The town may have got its name from the Black Warrior, an overladen whaling ship that sank here a year later, but that's only the most popular of theories.

During the season there are organized whale-watching trips, and an observation tower that guarantees at least a distant sighting. Although talk turns every year to restricting numbers or banning boats altogether, there are currently more tours and boat trips than ever. If you can take one, then do so – it's an exceptional experience, and many visitors actually get to touch the whales, which sometimes come right up to bobbing vessels after the engines are switched off. Whale-watching trips are run from Mario's; they charge around M$500 per person for a four-hour trip, including a complimentary drink or two. Malarrimo Eco-Tours (Tel:615/157-0100, Web: www.malarrimo.com ) also runs whale tours, as well as eight-hour tours to the Sierra de San Francisco to see cave paintings (Oct– Dec). If you are heading south, keep in mind that you will have two more opportunities to go whale watching: at San Ignacio and Bahía Magdalena.

To watch the whales from the shore, you'll need your own vehicle (preferably 4WD): head south from town until you see the park sign, from where a poor sand track leads 24km down to the lagoon. Midway there's a checkpoint where you must register your vehicle and its occupants, and at the park entrance a fee of around M$40 is charged. To see the whales you'll need to get up early or stay late, as they move out to the deeper water in the middle of the day.

If you want to stay in Guerrero Negro, you can choose from numerous hotels and motels strung out along the main drag, Zapata. Perhaps the best is the AMalarrimo, Zapata at Pipila (Tel:615/857-0250, Web: www.malarrimo.com ; Price: M$350-500), to the right as you enter town, with clean rooms with TV, RV spaces and a good seafood restaurant. Further into town, the Motel Las Dunas, Zapata s/n at División del Norte (Tel:615/157-0650; Price: M$250-350), is simple, clean and friendly, though sometimes a little noisy. You'll find much the same at Las Ballenas, Victoria s/n at Zapata (Tel:615/157-0116; Price: M$200-250), which offers a dozen or so basic rooms with TV; or, on the north side of Zapata, there's the good but overpriced and almost antiseptic Hotel El Morro, Zapata s/n at Victoria (Tel:615/157-0414; Price: M$250-350), whose rooms have fans and cable TV.

There are plenty of restaurants along the main street, though only a few ever seem to be open at any one time. Malarrimo, in the hotel of the same name, has good seafood and is another place to check about whale-watching trips; El Asadero Norteño serves meaty northern specialities; Mario's (Tel:615/157-0788), on the entrance to the highway, is good for breakfast and the basics; you can also try Don Gus, an agreeable place by the bus station where you can eat huge M$200 shrimp platters while gazing at one of the region's salt flats. Fresh produce can also be purchased from Supermercado La Ballena, on Zapata.

Buses from Guerrero Negro's bus station are irregular and often full upon arrival; buy your ticket at least a day in advance. The six services (one local) which head north to Tijuana (M$390), and the one to Mexicali, leave at night and early morning; the eight southbound services – two local, running all the way to La Paz (M$420) – depart either early in the morning or in the late afternoon and evening. The taxis at the stand in front of the blue and yellow Mercado Tianguis supermarket on Zapata adjust their rates depending on how long you'll have to wait for a bus – a ride to San Ignacio is about M$300 per person. A half-dozen Internet cafés (typically open Mon– Sat only; M$10–20/hr), such as Café Internet Las Ballenas, TechNet (open on Sun) and Cyber Guerrero Negro, line Zapata and the adjacent blocks between the Malarrimo and Hotel El Morro.