The phylogenetically young species community of San Antonio Cave (Oaxaca, Mexico) exemplifies the hypothesis that speciation of troglobites can occur in close contact with epigean predecessors. In a subterranean creek which continues outside with a rich epigean fauna, four troglobitic aquatic crustacean and one fish species (Rhamdia reddelli, Pimelodidae) were studied. Today not a single surface specimen can be found in the cave waters although several epigean species are troglophilic and/or are the ancestors of cave forms in other parts of Mexico. The absence of epigean invaders is attributed to the presence of specimens of some of the more aggressive and carnivorous cave species close to the cave entrance. Contrary to this it can be presumed that at the beginning of the troglobitic evolution the cave ancestral epigean forms were regularly invading the cave. It is assumed that photonegative behaviour played a role for the initial colonization of the cave but it is not of significance as a separating mechanism for the speciation process.