A cytological analysis of 37 experimental interspecific hybrids withinEpilobium sect.Epilobium, involving 23 taxa, has indicated that the CC chromosome arrangement differs from BB by a single reciprocal translocation and was probably, like AA and DD, derived from it. Preliminary results suggest that the CC arrangement may characterize the AsianE. cephalostigma andE. platystigmatosum, which do not closely resemble the species in which it had earlier been detected. The BB arrangement is shown to occur in all North American species of theE. palustre group, includingE. oregonense. Direct evidence on chromosome arrangement is now available for some 85 of the approximately 185 species of the section, and may be inferred for at least 25 more.