It is proposed that error patterns in acquisition of a second language can provide otherwise unavailable evidence for testing linguistic hypotheses about the second language itself. Three types of production and perceptual error patterns found in the learning of English by native Arabic speakers are outlined to support this suggestion. The error patterns discussed involve learner errors that can be seen as resulting from transfer of rules or principles of the learner's native language grammar, including the breaking up of consonant clusters, and two types of incorrect segmentation of strings into words, either by segmentation into two nonexistent words or mis-organization of syllables. (MSE)