Using unique personnel data from one Russian firm for the years 1997 to 2002 we study the size, development and determinants of the gender earnings gap in an internal labor market during late transition. The gap is sizable but declines strongly over the entire period. Gender earnings differentials are largest for production workers who constitute the largest employee group in the firm. Various decompositions show that these differentials and their dynamics remain largely unexplained by observable characteristics at the mean and across the wage distribution. Our analysis also reveals that the earnings differentials for production workers largely stem from job assignment, as women are predominately assigned to lower paid jobs. Earnings gaps within job levels are small and almost fully explained by observed characteristics.