Physical activity in Blaberus giganteus (L.) loosely follows a recurrent temporal pattern corresponding to the light–dark cycle. Differential haemocyte counts were made from serial blood samples taken before and after dark from individual adults in a brief attempt to determine whether the haemocyte population changed regularly also with this cycle. In 4 of the 16 individuals tested there was a substantial increase in the relative number of spherule cells and a somewhat proportional decrease in granular haemocytes after dark. In this species, these two haemocyte categories differ mainly in the size and composition of their cytoplasmic inclusions. The proportional change in their numbers is therefore considered to be the result of chemistry rather than a shift in populations of entirely separate kinds of cells. Retention of the two categories may be useful for indicating physiological changes in the blood. It seems possible that the cyclic change in numbers indicated here for these categories is more usual than shown, but is not revealed by the method employed. It seems sufficiently large however to warrant consideration in the design and interpretation of experiments relating to haemocyte populations.