Greyhound Bus Lines (U.S. tel. 619/515-1100, www.greyhound.com ) offers frequent service from San Diego (120 W. Broadway) to Tijuana  (both the downtown bus terminal and the Central de Autobuses (for southbound connections). Fares are US$14–19 one-way to the Tijuana airport and US$13–19 to downtown Tijuana. You can also take Greyhound from Los Angeles (1716 E. 7th St., U.S. tel. 213/629-8401, US$23–31 one-way to downtown TJ), which is a four-hour trip. There’s no extra charge for boxed bikes.
The easiest way to get from downtown San Diego to the border is the Blue Line Trolley (U.S. tel. 619/685-4900, www.sdcommute.com , US$2.25), which connects the Old Town Transit Center with the San Ysidro Transit Center (about an hour-long ride). Trolleys depart every 15 minutes (U.S. tel. 619/685-4900 for general info or U.S. tel. 619/233-3004 for schedule and stops, 5 A.M.–1 A.M. daily, “night owl” service 2–5 A.M. Sat.). You can bring your bike along.
The most economical bus from downtown San Diego to the border at San Ysidro is city bus 932 (U.S. tel. 619/685-4900, www.sdcommute.com , US$2.25), with daily departures every half hour, 6 A.M.–11 P.M. every day. The trip takes about 80 minutes one-way.
Once you’ve reached the border, Mexicoach buses (tel. 664/685-1470, U.S. tel. 619/428-9517, US$5) leave every 30 minutes 8 A.M.–9 P.M. for the Terminal Turística Tijuana (Av. Revolución btw Calles 6/7).
Greyhound (toll-free Mex. tel. 800/710-8819, www.greyhound.com.mx ) and its Mexican partner Crucero offer daily bus service from Los Angeles (US$36–48) and San Ysidro (US$25–34) to Calexico. A short walk from the station gets you through the border gateway and into downtown Mexicali.
Transportes Norte de Sonora (TNS, Tijuana tel. 664/688-1979) and Autotransportes Estrellas del Pacífico (www.estrellablanca.com.mx ) run long-distance express connections to Mexicali  (Calzada Independencia 1244, Col. Centro Cívico, tel. 686/557-2450), Tecate  (Juárez 321, Zona Centro, tel. 665/654-2343), Tijuana  (Lázaro Cárdenas at Río Alamar s/n, Col. La Mesa, tel. 664/621-2982), and Ensenada  (Calle 11 at Riverol, Zona Centro, tel. 646/178-6677) from various mainland destinations.
Budget travelers who like the idea of exploring Baja with an organized group might consider Green Tortoise (U.S. tel. 415/956-7500 or 800/867-8647, www.greentortoise.com ). It offers nine-day (from US$720) and 15-day (US$1,050) trips to Baja November–April that begin in San Francisco (pickups in L.A. and San Diego are possible) and go as far south as Cabo Pulmo . Prices include transportation, accommodations (on the bus), food, guided hikes, and side trips to the beach.