The aim of this exploratory study is to examine bilingual Russian–Hebrew-speaking children’s performance on the complex Case System in Russian. The speech of six early sequential bilinguals and three simultaneous bilinguals is analyzed for the quality and quantity of errors. Monolingual data came from two sources. The first source was the error rate of case and number by two normally developing monolingual Russian-speaking children, collected recently in the former Soviet Union. The second source was qualitative reports on error types made by monolingual children in the course of Case System acquisition. The following research questions were examined: (i) Are there differences between bilingual children and age-matched monolingual Russian-speaking children in Russian Case System acquisition? (ii) Are there differences between simultaneous and early sequential bilinguals in Russian Case System acquisition? Speech of bilingual children was recorded individually and monthly over a seven-month period, 20 minutes per month per child. Error analysis of the bilingual speech was conducted regarding the following target variables: noun oblique cases (Genitive, Dative, Accusative, Instrumental, and Prepositional), noun number (singular and plural), and the three declensions. The results show quantitative differences between simultaneous bilinguals, early sequential bilinguals, and monolinguals in Russian Case System acquisition.