Two groups of killifish, Fundulus heteroclitus, which were flown aboard Apollo-Soyuz, were subjected to postflight behavioral testing. The first group examined consisted of a graded series of embryos representing key developmental stages at orbital insertion (32-h, 66-h, and 128-h stages; pre-liftoff fertilization times). The tests revealed that the young adults from the flight 32-h stage had a significantly decreased positive geotaxis and increased negative phototaxis when compared with ground controls. These findings suggested a greater sensitization of the least-developed flight stage to post-hatching environmental influences. The second group consisted of 21-d old juveniles which were subjected to light orientation tests soon after recovery. No significant differences were detected.