1. The chromosome numbers of 51Miridae and nineNabidae (Hemiptera-Heteroptera, Cimicoidea) are given. 2. InMiridae, Deraeocorinae, Bryocorinae andMirinae have a type 2n=34 and Phylinae 2n=32. Dicyphinae has 2n=48 and Orthotylinae 2n=24. The basic mirid caryotype is 32A+X+Y. 3. The caryotype ofDicyphinae supports the view that the group warrants subfamily status. 4. Phylus melanocephalus (2n=18) is unique amongst the mirids examined and could perhaps be retaining a primitive pro-mirid chromosome number. 5. InNabidae, Nabinae have a type 2n=18. 6. The species pairNabis rugosus andN. ericetorum are characterised by the same kind of deviation from family caryotype. 7. Himacerus mirmecoides has double the family complement of autosomes. 8. The basic cimicoid caryotype is, it is suggested, 2n=16A+ X+Y.