This study investigated the acquisition of the comprehension of overt morphological casemarking by adult native speakers of English who were learning Russian or German as a secondlanguage (L2). The Russian case-marking system is more complex than the German system, butit almost always provides the listener with case inflections that are reliable cues to sentenceinterpretation. Two approaches to learning of inflectional morphology were contrasted: therule-based approach, which predicts that learning is determined by paradigm complexity; and theassociative approach, which predicts that learning is determined by the cue validity of individualinflections. A computerized picture-choice task probed the comprehension of L2 learners byvarying the cues of case marking, noun configuration, and noun animacy. The resultsdemonstrated that learners of Russian use case marking much earlier than learners of Germanand 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 wasdeveloped that correctly simulated the obtained results. Together, these findings support the viewthat adult L2 learning is associative and driven by the validity of cues in the input.