The suborder Cyprinodontoidei (order Cyprinodontiformes) of Euteleostei includes killifish of which, in spite of their popularity among aquarists, only a few have been studied for their hemoglobins. Here, the electrophoretic patterns are presented of 21 out of 497 species. They all show high multiplicity and no evidence of polymorphism, indicating that the patterns are selectively conservative. Although some generalizations may be made, no functional explanation for the stability of such complex hemoglobin mixtures exists so far.