This article provides a detailed analysis of generation plant operating cost efficiency over the period 1988-1997. I estimate a stochastic frontier cost function together with a model of plant inefficiencies to measure the operating efficiencies of individual plants and identify factors that are associated with variations in plant efficiencies. I find evidence that plant efficiencies are associated with capacity utilization of the plant, the number of plants under utility management, and ownership form. I also find that regulatory restructuring activity in certain states is associated with improvements in plant operating performance. The purpose of this article is to provide a detailed analysis of the operating cost efficiencies of electricity generating plants over the period 1988-1997. I estimate the operating efficiencies of fossil-fueled steam generating plants and examine factors that influence plant operating performance. I use the estimation procedure of Battese and Coelli (1995) to simultaneously estimate a stochastic frontier cost function and a function relating plant efficiency levels to plant-specific factors and other explanatory variables. A comprehensive data set allows me to investigate the influence of capacity utilization, ownership form, and utility experience on plant efficiency levels. Since significant regulatory change in the electricity industry had begun during this time period, I also investigate the impact of state-level regulatory change on plant operating efficiency. A number of authors have analyzed the efficiencies of electricity generating plants relative to a best practice frontier, including Kopp and Smith (1980), Schmidt and Lovell (1979), Stevenson (1980), Cote (1989), Reifschneider and Stevenson (1991), Hammond (1992), Pollitt (1995), and Fire, Grosskopf, and Logan (1983, 1985). These studies find considerable evidence of inefficiency in electricity generation during the 1960s and 1970s. The present study provides more recent information regarding generating plant efficiencies during a period of significant regulatory and structural change. Studies investigating the determinants of power plant departures from the frontier have focused primarily on the effects of ownership mode. Pollitt (1995) provides a comprehensive review of these studies. Reifschneider and Stevenson (1991) examine the impact on cost efficiency of a number of variables, including the capacity utilization of the firm's generating facilities. More recently, Knittel (2001) examines the impact of incentive regulation on plant efficiency levels. Steiner