The structure and the construction of the bilingual mental lexicon are here the focus of attention. The paper explores the issue of whether the bilingual mental lexicon works in the same way as the monolingual lexicon, whether there are differences between the storage systems of the two sets of lexemes. Based on a Hungarian word association test carried out among 90 bilinguals, of whom 50 are early bilinguals and 40 late bilinguals, the main goal of the present paper is to find some evidence concerning the storage hypothesis. Bilinguals have turned out to have one common semantic representation but two sets of words that are stored in one common lexicon. There are significant differences in the construction of the mental lexicon according to the age of becoming bilingual. Early bilinguals are more likely to store the information in a paradigmatic way whereas late bilinguals tend to store more syntagmatic relations. Particular emphasis is put on the question of how the word classes of the stimulus words control those of the responses, i.e. whether the word class has a determining role in the storage of lexemes. By and large all the word classes tend to preserve their word classes in the responses, but the nouns have proved to be the ones that are most likely to show paradigmatic links. The largest number of syntagmatic links was shown in verbs. Word class bound responses were in even larger numbers if the bilinguals gave the answers in their other language.