In the present study, the natural and simultaneous first language acquisition of a German-Italian bilingual boy and an Italian monolingual girl from 1;8 to 2;1 are analyzed and compared. The investigation focuses on the rate of acquisition and the size and composition of the productive lexicons. At the end of the observation period, the bilingual boy has acquired twice as many words as the monolingual child. The rate of acquisition of both children markedly increases (vocabulary spurt) after their lexicons have reached the size of 50 words. The data reveal a noun preference (noun bias) in the lexicons of the bilingual and the monolingual child. The comparison of the Italian lexicons of the bilingual child and of the monolingual child do not show striking differences regarding word categories. In both lexicons, common nouns, proper nouns, and onomatopoeic words are the most important word categories, whereas verbs play a marginal role. (Contains 8 figures, 3 tables and 3 notes.)