yc4 is an unstable yellow-in-the-dark mutation found in the unicellular green alga Chlamydomonas reinhardtii. yc4 reverts to the wild-type green-in-the-dark phenotype at the frequency of 1x10⁻⁴ revertants per cell division. We have tested whether mutation and reversion of yc4 is due to the insertion and excision of the C. rein-hardtii transposon TOC1. Southern blots showed differences in the distribution of TOC1 elements when the original mutant yc4 strain, which contained over 30 TOC1 elements, was compared with green-in-the-dark-revertants. However, analysis of the yc4 mutation in a different genetic background containing half the number of TOC1 elements showed that reversion of yc4 is not associated with loss of TOC1 elements.