In the laboratory the influence of light, temperature and relative humidity upon the behavior of L. sericata (Meig.) was investigated. The flies were kept under different conditions: .male.male. and .female.female. isolated or together, fed with sugar only or in addition to meat, continuous feeding or insertion of a hunger period of 24 h before the test. In all variations the flies had a strong positive phototaxis. This was increased by an additional feeding with meat. When fed with sugar only, a hunger period reduced the positive reaction in .male.male. but was without influence in .female.female. In these hunger tests the meat-fed flies increased the positive reaction during the 1st wk, but showed a strong reduction in the 2nd wk, compared with normally fed flies. .male.male. and .female.female. kept together with the possibility of copulation and egg laying were more positively phototactic in the 1st wk and lower in the 2nd wk, compared with isolated flies. In nearly all variations the tendency of preferring the warmer areas of the test equipment was increased during the lifetime of the flies. A preference for a higher humidity was noticed in hungry flies, while normal fed flies did not show such a reaction. In the tests with different humidities the highest percentage of flies had positive or negative reactions, while in the light tests the number of flies without clear decision was highest. This leads to the suggestion that the humidity has a pronounced value in the orientation of L. sericata.