Six synthesized consonant-vowel syllables, three containing the phoneme /b/ in different vowel contexts and three the phoneme /d/, were presented randomly to developmental dysphasics and normal children. The ability to recognize that these six acoustically different stimuli shared two common phonemic categories (perceptual constancy) was investigated using nonverbal operantly conditioned response techniques. Results showed that although several children in both groups had difficulty with the task, the dysphasic group's performance was significantly poorer than the controls. Whereas the normal children improved significantly with age, the dysphasics did not. The results of this study suggest that speech perception, rather than being fully developed in infancy, changes throughout language development. By using procedures which have proven suitable for testing infants, with young children at various stages of language development, more might be learned about how the acoustic signal is encoded into speech and language and how this encoding changes throughout development or is disturbed in language disorders.