This article reports the results of speech error elicitation experiments investigating the role of two consonant co-occurrence restrictions in the productive grammar of speakers of two Ethiopian Semitic languages, Amharic and Chaha. Higher error rates were found with consonant combinations that violated co-occurrence constraints than with those that had only a high degree of shared phonological similarity or low frequency of co-occurrence. Sequences that violated two constraints had the highest error rates. The results indicate that violations of consonant co-occurrence restrictions significantly increase error rates in the productions of native speakers, thereby supporting the psychological reality of the constraints. (Contains 14 tables and 6 footnotes.)