This paper reports on the acquisition of the voicing contrast in Mexican-Spanish word-initial stops. In Study 1, three Spanish-speaking monolingual children were recorded every two weeks for seven months, beginning when the children were about 1;7. In Study 2, four monolingual children about 3;10 were recorded once or twice. Two analyses were done. Instrumental analysis of the stop productions revealed that not even by age 3;10 were the children consistently distinguishing between voiced-voiceless stop cognate pairs on the basis of adult-like voice-onset time characteristics. The spirantization analysis, however, more clearly revealed the children's phonological knowledge. Discussion focuses on the implications of the data for phonological development in general and for the phonological description of voicing in Spanish. (Author)