(1) Survival and duration of development of two Tribolium castaneum strains were not the same when reared in environments of different shapes under the same external conditions. (2) The bb strain survived better in jars than vials. The ++ strain survived better in vials than in jars of type 3. (3) Both strains (as well as their hybrid (+b)) developed faster in vials (type 1) than in shallow jars (type 2). Both strains developed faster in jars of type 3 than in vials, at the low density. At the high density, the strains reacted differently: ++ developed faster in vials, but bb developed faster in jars. (4) Cultures of "selected" strains followed the population trends they exhibited in the selection experiments from which they came. (5) In both densities and in all environments, mean individual weight of ++ adults was always higher then that of bb, suggesting that this character is, at least in part, genetically determined. (6) The differences in survival and developmental period in environments of different shapes may be due to microclimatic differences between environments and to changes produced in them by the developing immatures.