Invertebrates that contain endosymbiotic chemoautotrophic eubacteria are widely distributed in a variety of reducing marine habitats, including deepsea hydrothermal vents. The mechanisms of symbiont transmission in these invertebrates are not understood. To test the hypothesis that symbionts are transmitted via the eggs of hosts, we used group-specific hybridization probes complementary to 16S ribosomal RNAs (rRNAs) to look for symbionts in eggs and ovaries. 16S rRNA sequences were examined for domains unique to the symbionts of three vent animals: Calyptogena magnifica, Bathymodiolus thermophilus, and Riftia pachyptila. Three 16S rRNA-directed oligodeoxynucleotide hybridization probes (CG-1255R, RP-1243R, BT-1255R) specific for these endosymbionts were synthesized and evaluated by dot-blot hybridization. At higher stringencies, all three probes showed a high degree of specificity for their target endosymbionts rRNAs. The probes were also used as polymerase chain reaction (PCR) primers for detection of the symbiont 16S rRNA genes in genomic DNA isolated from host tissues known to contain symbionts. All three symbiont-specific probes were highly sensitive and specific as PCR primers; they successfully amplified 1 pg target DNA. However, all amplifications of extracted egg DNA from the vestimentiferan R. pachyptila with either universal eubacterial (Eub A/B) or the Riftia symbiont-specific (RP-1243R/Eub B) primer sets were unsuccessful. Nonradioactive in situ hybridizations were performed on ovarian tissue from the vestimentiferan Ridgea piscesae using RP-1243R, 3' end-labeled with digoxigenin-11-dUTP (Boehringer Mannheim). The probe was subsequently detected with an alkaline phosphatase-conjugated immunoglobulin G antibody specific for the digoxigenin moeity. The probe bound only to the tissue of R. pisceasae coincident with the known location of symbiont cells and was not detected in any region of the ovary. These data indicate that transovarial symbiont transmission in the vestimentiferans does not take place and that symbiont acquisition is probably a post-spawning event.