This paper analyzes the phenomenon of case-marking errors in L2 Russian from the standpoint of the psycholinguistic theory called Incremental Procedural Grammar. The main concepts of this theory are: (1) parts of a sentence are processed incrementally, relatively independently and simultaneously; (2) articulation of the sentence may start when the whole sentence has not been processed yet to insure fluency; (3) speech production is lexically driven . The paper shows how Second Language Acquisition (SLA) concepts, such as interlanguage, language transfer and error analysis fit into this relatively new theoretical approach. The research, supported by examples from the spontaneous, oral speech of one L2 learner, shows that the errors in case-marking in L2 Russian are caused by the transfer of the L1 production rules, interlanguage production rules and slowness in the usage of newly developed production rules of L2 within a speech production system as outlined in Incremental Procedural Grammar.