Using written texts elicited from students with different proficiency levels, we studied the acquisition of nominal cases in Russian as a second language. We established the order in which cases were acquired (nominative, locative, accusative, genitive, instrumental, and dative), as well as certain characteristics of their acquisition trajectories. We argue that the order of acquisition is determined by two groups of factors: how essential a given case is for successful language use and how complex it is, both semantically and morphologically. Many important differences between first and second language acquisition were identified. The maturation of the case system was observed both in the number of correct forms participants produced and in error rates. The distribution of correct forms changed, gradually approaching that found for native speakers. With growth in proficiency, the incidence of different error types declined, and the relative frequency of errors more characteristic of first language acquisition increased.