This study investigated the acquisition of the comprehension of overt morphological case marking by adult native speakers of English who were learning Russian or German as a second language (L2). The Russian case-marking system is more complex than the German system, but it almost always provides the listener with case inflections that are reliable cues to sentence interpretation. Two approaches to learning of inflectional morphology were contrasted: the rule-based approach, which predicts that learning is determined by paradigm complexity; and the associative approach, which predicts that learning is determined by the cue validity of individual inflections. A computerized picture-choice task probed the comprehension of L2 learners by varying the cues of case marking, noun configuration, and noun animacy. The results demonstrated that learners of Russian use case marking much earlier than learners of German and that learners of German rely more on animacy to supplement the weaker case-marking cue. In order to further explore the underlying mechanisms of learning, a connectionist model was developed that correctly simulated the obtained results. Together, these findings support the view that adult L2 learning is associative and driven by the validity of cues in the input.