The development of the click sounds of sperm whales (Physeter catodon) has been investigated through comparisons of these vocalizations from calves of different sizes. The observations include sounds from four small stranded calves held for short periods in aquaria at Miami, Florida, and Seattle, Washington, and in a bay on Long Island, New York. These vocalizations were compared with those of larger calves encountered at sea. All the calves produced typical sperm whale sounds -- clicks with broadband spectra, often produced in short series. Vocalizations from the smaller calves sometimes included slightly noisy, tonal components -- similar noisy click sounds were also occasionally heard in the presence of calves at sea, and they were interpreted as the result of improperly formed clicks. The smallest calves produced few temporally repetitive click patterns, but the larger (older) ones produced sequences with stereotyped “coda”-like temporal patterns. The use of such patterned click sequences increased with the apparent age of the calves. In the larger calves, the sounds appeared more organized in sequences similar to the communicative signals of adults. None of the calves appeared to use their sounds in ways that were related to echolocation.