Twenty-one species of Australian lygaeids belonging to six subfamilies have been examined cytologically. The two species of Lygaeinae both have 2n B = 14 including an XY sex chromosome pair. While all the remaining species were also XY in the male, diploid numbers of 12, 14, 16, 20 and 22 were found. which included a minute m-chromosome pair which is not represented in the Lygaeinae. The observed variation in chromosome number and size provides supporting evidence for the occurrence of both fusion and fission within the Heteroptera.