Males were subtracted from or added to existing populations of C. puella beside small ponds in field experiments at Woodwalton Fen, Cambridgeshire, United Kingdom (1992, 1993). When insects were subtracted the population returned to the original level within IVi h. Addition of insects from elsewhere raised low populations to a certain level, but did not raise populations already at that level. C. paella do not fight, but the pursuit of one male by another can lead to the pursued one leaving the water. Whether or not this is the mechanism controlling male population density by water, clearly the behaviour of the insects is at least partly responsible. The quasi-territorial behaviour in C. paella operates at low as well as high population densities. It enables a higher steady population density to occur than is found in the truly territorial species so far studied. An interesting and unexplained anomaly of population density in C. paella is described.