The empirical modeling of imperfectly competitive markets has been constrained by the difficulty of obtaining micro data on individual producer prices, outputs, and costs. In this paper we utilize micro data collected from the 1977 Census of Manufactures to study the determinants of plant-level output prices among U.S. bread producers. A theoretical model of short-run price competition among plants producing differentiated products is used to specify reduced-form equations for each plant's price and output. Estimates of the reduced-form equations indicate that the main determinants of both the plant's output level and output price are the plant's own cost variables, particularly its capital stock and the prices of material inputs. The number of rival producers faced by the plant, the production costs of these rivals, and the demand conditions faced by the plant play no role in price or output determination. The results are not consistent with either oligopolistic competition or monopoly behavior, but rather are consistent with price-taking behavior by individual producers combined with output quality differentials across producers.