Abstract.1. The population densities ofDrosophila obscuraandD. subobscurain mixed deciduous woodland were investigated by multiple recapture techniques, marking flies with micronized fluorescent dusts.2. Approximate densities per 100 m2were as follows: September 1974:D.subobscuramales 5,D.subobscurafemales 31. April 1975:D.obscuramales 1,D.obscurafemales 2;D.subobscuramales 0.3,D.subobscurafemales 1. June 1975:D.subobscuramales 6,D.subobscurafemales 9.3. Actual estimates have been compared using relative estimates derived from a reanalysis of Shorrocks's (1975) data.4. Throughout most of the yearD.subobscurawas numerically dominant, rising to a peak of 750/100 m2in the winter; but in late springD.obscurawas dominant, having reduced winter mortality by diapausing.5.D.obscura's density was comparable with those of its Nearctic relatives;D.subobscura's was considerably higher.6. The effects of previous workers' approximations on the estimation of population parameters have been examined, and possible theoretical consequences have been indicated.