In a previous paper (Crofton, 1954) it was shown that the rise in egg counts occurring in ewe flocks in spring was related to the time of lambing. It was further shown that the rise in individuals occurred about 6 weeks after parturition and was normally of short duration. While the mean time of increased egg count for each individual was about 2 weeks, the increased egg count in the flock was spread over a period equivalent to the length of the lambing period. It is clear that the apparent rise in the flock as a whole will be greatest when all the flock lambs within a short period. With greater scatter of times of parturition the flock rise will be smaller but will extend over a greater period. This association of rise in egg count with time of parturition is obviously one of considerable selective advantage, in so far as an increase in infective stages will occur at a time when the susceptible lambs have a reduced milk intake and an increased grass intake. The synchronization of availability of infective stages and the occurrence of susceptible individuals is further emphasized by the relation of the period of increase to the time of birth of the lambs, the duration of the period of increase in the flock being directly proportional to the duration of the lambing period. This type of ‘escapement’ control is seen in so many other host-parasite relationships that it makes considerable appeal on general grounds. Other workers, Morgan, Parnell & Rayski (1951), have, however, related the rise in egg counts to climatological factors, but climatic control would not give the close synchronization described above.