Perhaps the most disturbing trend in American public transit during the 1990's has been the alarming ridership declines among the nation's largest operators. Between 1989 and 1993, annual ridership in the nation's largest urban areas declined by 680 million passengers, almost 10 percent, while ridership in smaller cities grew by a modest 12 million. Such aggregate figures mask even more troubling ridership declines among the ten largest public transit systems in the U.S., which saw transit use plummet by almost 11 percent, or 537 million annual passengers in four years. This ridership loss nearly equaled the entire 1993 ridership of the nation's second (Chicago) and tenth (Baltimore) largest transit systems combined. In contrast to the sharp contractions on the oldest and largest systems in the U.S., other transit operators serving the same metropolitan areas as the "Top-Ten" added over 57 million annual riders, or about 7 percent between 1989 and 1993, though these gains were not enough to offset the ridership losses on the ten largest systems. This paper examines the dimensions of recent ridership losses on the nation's largest public transit systems and concludes that the ridership losses on the Top-Ten systems are due largely to two factors: (1) a cost-revenue squeeze that has forced service cutbacks on some Top-Ten systems and (2) a substantial decline in service productivity that has cut the effectiveness the service that remains.This overview section is followed by a case study of the nation's seventh largest transit system: the San Francisco Municipal Railway (Muni). This case study shows that declining revenues (both from fares and subsidies) have forced Muni to cut service, largely in the form of missed runs, and that the combined effect of decreasing service hours and declining service quality has been lost patronage. This case study also shows that Muni's financial picture is expected to worsen in the coming years, which will likely accelerate patronage loss.The recent experience of Muni contrasts sharply with suburban transit operators in the San Francisco Bay Area, many of which have added service and riders during the early 1990's. Short of substantial increases in subsidies or substantial reductions in operating costs, the recent combined ridership losses experienced by the nation's ten largest transit systems will likely continue, and perhaps worsen, through the mid-1990's.