Drawing on recent psychological and neurological research on how individual differences might interact with learning a particular language, the study examines how psycholinguistic research and theory can help in assigning military personnel to language training and to a given language. Using the Defense Language Institute's Defense Language Aptitude Battery (DLAB) for reference, five categories of language difficulty are described, based on degree of difference from English. Research on individual differences in language learning is reviewed to identify potential methods of improving the DLAB to measure individual traits allowing a student to excel at certain languages or language skills. Language learning presents difficulties in orthography, phonology, lexicon, morphosyntax, syntactic processing, language use, and language learning strategies. For each there is rich psycholinguistic literature that can be used as a basis for additional measures of learner abilities. In practice, it may be useful to increase emphasis on prediction of second language learning from first language skills. It may also be easier to measure skill at learning and processing certain aspects of language. It is concluded that more finely tuned psycholinguistic measures of language learning skills are needed to predict problems caused by individual differences. (Contains 111 references.) (MSE)