New chromosome rearrangements were found in 10% or more of mitotically stable transformants. This was shown for transformations involving a variety of different markers, vectors and recipient strains. Breakpoints were randomly distributed among the seven linkage groups. Controls using untransformed protoplasts of the same strains contained almost no rearrangements. A study of molecularly characterized Am+ transformants showed that rearrangements are frequent when multiple ectopic integration events have occurred. In contrast, rearrangements are absent or infrequent when only the resident locus is restored to am+ by a homologous event. Sequences of the transforming vector were genetically linked to breakpoints in 6 of 10 translocations that were examined using Southern hybridization or colony blots.