Cytochemical methods and electron microscopy were used to study changes in the chemical composition of nuclear, nucleolar and perinuclear bodies during the early stages of oocyte development in Poecilia formosa, an apomictic species of fish that produces only female offspring. Prominent accumulations of ribonucleoprotein (RNP) occur in nucleoli and appear on either side of the nuclear envelope during diplotene. In certain planes of section, RNP material seems to be in transit across this interface. En bloc acid extractions or RNAase treatment abolished basophilia and markedly reduced the electron density of both nucleoli and cytoplasmic nucleolar-like bodies. DNA-specific fluorescent probes such as mithramycin failed to reveal nucleolar cores in poeciliid oocytes, although the same procedures showed unequivocal localization of GC-rich DNA cores within multiple nucleoli of diplotene oocytes from Xenopus laevis or the rainbow trout, Salmo gairdneri. Also, cytological hybridization studies, utilizing [3H]rRNA as a probe for nucleolar DNA, failed to show localizations of silver grains above nucleoli in autoradiograms of poeciliid oocytes. Feulgen-stained pachytene oocytes of P. formosa have twice the number of chromosome strands seen in similar stages of oocytes from two, related bisexual species, P. mexicana and P. latipinna. Although the bivalent nature of these chromosomes could not be resolved with the light microscope, configurations resembling, but not identical to, synaptonemal complexes were identified by electron microscopy.