The adaptive significance of the phenological relationships between predators, aposematic species, and their Batesian mimics has received surprisingly little attention and has more often been the subject of speculation than of systematic observation or experimentation. It was once believed that models and mimics must coincide in both place and time if the mimics were to benefit from their mimesis. Although many mimics no doubt do coincide in time with their models, closer examination of the properties of Batesian complexes reveals that mimics actually have considerable latitude in their phenological positions with respect to the model and that some do not coincide temporally with their models. Thus, a mimic may shift its timing to coincide more closely with resources such as oviposition sites or plants that provide pollen and nectar. Furthermore, as explained below, asynchrony with the model may sometimes actually enhance the effectiveness of the mimicry itself. Therefore, existing mimics may tend to shift to phenological slots that enhance the protective value of their mimicry, and non-mimics with phenologies that are particularly favorable to mimicry may be more likely to evolve mimicry than are other nonmimics. The following discussion considers mainly Batesian complexes, of the northern temperate zone, that consist of stinging bumblebees or wasps as models, their dipteran mimics, and insectivorous birds. This does not imply that phenological considerations may not be important in the tropics or with other Batesian or Mullerian complexes.