Cyprinodon variegatus and Fundulus grandis, two species of Cyprinodontid fishes extensively studied and used in toxicological and biological investigations, are reviewed and compared as laboratory test animals. Their ecology and general biology, and suitability for various types of experimentation are examined. A laboratory system for exposing critical life stages (e.g., embryos, fry, juveniles) of these species to suspect carcinogens is described. A discussion of the use, findings and potential of these species in oncological studies and carcinogen assays is presented, particularly in regard to responses to three known or suspect carcinogen chemicals (e.g., trifluraline, benzidine, and aflatoxin). Finally, advantages and disadvantages or special problems in using the species as carcinogen test animals are reviewed.