The results of two experimental studies of interference in German-Swedish bilingual and trilingual high school students are presented. Both were developmental studies with length of residence in Sweden as the main independent variable. The purpose was to follow the developmental changes in intra- and interlingual interference and to find out if those interference patterns are related to intelligence as measured by a non-verbal test. In one study, vision tasks were used and in the other, dichotic auditory translation tasks were used. The results of the first study indicate that the differential pattern of interference is mainly determined by language dominance. Equivalent amounts of intra- and interlingual interference occur in subjects with roughly equivalent skills in each language, and higher interference from one language occurs when that language is dominant. The interference pattern is not task dependent, but the task difficulty appears to determine the point of language balance. Results of the second study suggest a pattern of interference very similar to that of the first study. It also found that concrete sentences consistently produced fewer errors than abstract sentences, which could be attributable to a number of factors or processes. (MSE)