The fis phenomenon is a phenomenon of child language acquisition that demonstrates that perception of phonemes occurs earlier than the ability of the child to produce those phonemes.

The name comes from an incident reported in "Psycholinguistic Research Methods" by J. Berko and R. Brown in Handbook of Research methods in Child Development, edited by P. Mussen (New York: John Wiley, 1960). A child referred to his inflatable plastic fish as a fis. However, when adults asked him, "Is this your fis?" he rejected the statement. When he was asked, "Is this your fish?" he responded, "Yes, my fis."

This shows that although the child could not produce the phoneme /?/, he could perceive it as being different from the phoneme /s/. This has important implications for the acquisition of phonology. In short, it means that children have more, not fewer, phonological processes (or rules) applying in their speech than adults, and that part of the task of acquiring a language is figuring out which processes to allow to apply and which to suppress.