Six squirrel monkeys (Saimiri sciureus) were studied for their vocal development over the first 20 months of life, using a multi-parametric acoustic analysis. Four of the animals were normally raised, one animal ('Kaspar Hauser') was deprived of adult species-specific calls, one animal was congenitally deaf. The study showed that in all 12 call types under investigation, age-related changes in one or the other acoustic parameter occurred. The degree to which vocal structures changed, was call type-specific, with some calls showing dramatic changes and others showing only minor changes. Most changes occurred within the first four months; some lasted up to 10 months. All call types showed a high variability throughout the 20 months study period. Only five of the 12 call types showed a reduction in variability over time. Both acoustically deprived animals remained within the variability range of the normally raised animals, suggesting that the ontogenetic changes found were mainly maturational.