We develop a model of phonological contrast in natural

language. Specifically, the model describes the maintenance of

contrast between different words in a language, and the elimination of

such contrast when sounds in the words merge.  An example of such a

contrast is that provided by the two vowel sounds "i" and "e", which

distinguish pairs of words such as "pin" and "pen" in most dialects of

English.  We model language users' knowledge of the pronunciation of a

word as consisting of collections of labeled exemplars stored in

memory.  Each exemplar is a detailed memory of a particular utterance

of the word in question.  In our model an exemplar is represented one

or two phonetic variables along with a weight indicating how strong

the memory of the utterance is.   Starting from an exemplar-level

model we derive integro-differential equations for the evolution of

exemplar density fields in phonetic space. Using these latter

equations we investigate under what conditions two sounds merge

causing words to no longer contrast. Our main conclusion is that for

the preservation of phonological contrast, it is necessary that

anomalous utterances of a given word are discarded, and not merely

stored in memory as an exemplar of another word.