It has frequently been observed that individual differences (IDs) in behavior can be inherited; e.g., Tryon (11) has reported on the inheritance of maze-learning ability, and Kallmann and Baroff (5) on the inheritance of behavior pathologies. The present paper extends the study of the inheritance of IDs in behavior to a part of the phylogenetic series at which experimental behavior genetic (BG) analysis is feasible, viz., the genus Drosophila. The behavior chosen for BG analysis is the reaction to light, phototaxis--an apparently innate or unconditioned response. Taxes have the advantage of representing relatively conslant S-R relationships: the repeated presentation of a single stimulus value appears to elicit, depending on the method of measurement, either a characteristic response or a characteristic probability of response. Both the characteristics of the response and the probability of the response have been shown to vary as a function of two parameters, the value of the stimulus presented and the strain of organisms stimulated (1, 8). Brown and Hall have measured strain differences in phototaxis. The immediate purpose of the present study is to measure IDs in, and to estimate the heritability of, phototaxis within a single strain. (Roughly, "heritability," h-super(2), refers to that portion of the total variance due to additive genetic causes [6, p. 111].) This is one of several studies of Drosophila behavior in which an experimental attack is being made on the long unresolved question of whether abilities are under the control of one general factor (9) or many specific factors (10). (PsycINFO Database Record (c) 2006 APA, all rights reserved)