Somatic tissues in mature copepods are determinate in growth, not undergoing mitosis; cell number remains constant throughout adult life. Here we report evidence of polyploidization for eight species of cyclopoid copepods. Using static Feulgen-DNA cytophotometry, we measured individual somatic nuclei in populations of Eucyclops ensifer from Brazil and of Eucyclops agilis from Ohio, U.S.A. Small but potentiality significant percentages of the adult somatic cells in these species, as well as in Bryocyclops caroli, Halicyclops tagea, Macrocyclops albidus, Mesocyclops edax, Mesocyclops thermocyclopoides, and Thermocyclops decipiens, contained at least twice (4C) the amount of DNA found in their diploid (2C) cells. These species have 2C DNA values that are representative of the range of genome sizes in cyclopoid copepods which, as a group, have much smaller genomes than calanoid copepods. Polyploidy may be a previously unrecognized mechanism whereby copepods alter the DNA content or nucleotype during specific stages in development. DNA reduplication preceding a cycle of endomitosis may result in a doubling of the functional genome, thus providing additional template for mRNA transcription related to specific functions.