A study investigated patterns in phonological errors occurring in the speaker's second language in both formal and informal speaking situations. Subjects were three adult learners of English as a second language, including a native Spanish-speaker and two Asians. Their speech was recorded during diagnostic testing (formal speech) and in everyday contexts (informal speech) and analyzed for phonemic interference from the first language using principles of both contrastive analysis theory and error analysis theory. Results support the use of contrastive analysis for this purpose, showing variability in error patterns linked to first language. Further research with a larger sample and a wider variety of first languages is recommended. Contains 31 references. (MSE)