In Chlamydomonas reinhardtii, we have found that linkage groups XII and XIII define only a single linkage group and that linkage groups XVI and XVII also define a single linkage group. The interdigitation of the genetic maps of linkage groups XII and XIII and of XVI and XVII is presented. At present, 17 linkage groups that display Mendelian segregation have been identified in C. reinhardtii.