Photoresponses of isofemale lines of Drosophila pseudoobscura, D. persimilis and D. miranda captured in two locations were compared in two types of experimental apparatus, an open field and a Y-tube. Photoresponses were significantly more negative in the open field than in the Y-tube. Responses in one apparatus were unrelated to the responses in the other. The sibling sympatric species from each location could be distinguished by their light responses irrespective of the experimental design. The populations of D. pseudoobscura from the two locations were significantly different in both designs. These behavioral differences must be attributable to different genetic determinants. INTRODUCTION Rockwell and Seiger (1973a) in a review of photoresponse in the genus Drosophila noticed that in the past experimental designs tend to fall into one of three classes. In Class I, photoresponse is measured as the movement or distribution of the individuals relative to a directed light source. For example, in the very first paper that used fruit flies for research, Carpenter (1905) measured photoresponse in terms of movement toward a light source and distribution of flies in a light gradient. In Class II, photoresponse is measured by the distribution of flies in an open field in which areas of the field differ in quality or intensity of light. Movement of the flies is in a plane perpendicular to the light source (Rockwell and Seiger, 1973b; Rockwell et al., 1975). In Class III, photoresponse is measured by the frequency of choice flies make when they reach a juncture of two light intensities. The two lights may be provided by light directed at the choice point (Hirsch and Boudreau, 1958; Pittendrigh, 1958) or by a light shining perpendicular to the plane of movement (Lewontin, 1959; Dobzhansky et al., 1969; Seiger and Seiger, 1979). It is difficult to make generalizations about photoresponse using different experimental designs. It may be that different experimental designs elicit different photobehavior from any given population of Drosophila. This possibility was studied in the current research. MATERIALS AND METHODS Drosophila pseudoobscura, D. persimilis and D. miranda are sibling species of the obscura species group within the subgenus Sophophora of the genus Drosophila (Dobzhansky and Epling, 1944). The isofemale lines (each started from one female caught in the wild) used in this study were kindly provided by Dr. Francisco Ayala, University of California, Davis. This study of five isofemale lines each of D. pseudoobscura and D. persimilis from MacDonald Ranch, Calif., and D. pseudoobscura and D. miranda from Mather, Calif., began with the F2 generation and took somewhat less than a year to complete. All strains were cultured in half-pint milk bottles using a cream-of-wheat and raisin medium at a temperature of 20 zt 1C under constant illumination of 3.0 ergs/cm2-sec.