In an earlier paper (Hassell 1968) the behavioural responses of the adult population of Cyzenis albicans (Fall.) at Wytham Woods, Berkshire towards uneven spatial distributions of hosts were considered. To explain the part played by Cyzenis in the natural control of the winter moth population it is the intergeneration relationships that are most important. Only from these can firm conclusions be drawn about the ability of natural enemies to regulate their hosts or prey. An intergeneration relationship is detected when a plot of the yearly mean percentage host mortality (or k-values) caused by the enemy population against the yearly mean host density (or its logarithm) reveals a linear, curvilinear or some form of spiral relationship. Each of these has, of course, very different implications. A curvilinear or rectilinear relationship indicates a superproportional or subproportional relationship depending on whether the slope is positive or negative. Such relationships imply a relatively constant parasite or predator density. A tendency for 'spiralling' to occur when the points are joined serially implies some form of delayed relationship, resulting from changes in the parasite or predator density, which can be caused only by changes in their reproductive rate and/or survival (Hassell 1966). Cyzenis is a specific parasite of the winter moth with a single annual emergence moreor-less synchronized with the availability of host larvae, and so may be expected to play an important part in the control of the winter moth population at Wytham. However, the intergeneration relationship between Cyzenis and the winter moth (Varley & Gradwell 1968) reveals no sign of delayed density-dependency, and the variation in the winter moth mortality due to Cyzenis is clearly insignificant when compared with the variation in generation mortality of the winter moth. The mean density of adult Cyzenis in the area of oak studied at Wytham since 1950 is 0 96/m2 (having varied between 7-2 and 0 1/Im2). In only three of these years has there been more than 700 parasitism of winter moth larvae. In contrast, in Nova Scotia, Canada, where Cyzenis was first introduced in 1954 (Graham 1958), adult parasite densities rapidly increased and in 1960 were causing a mean of 58% parasitism (Embree 1965, 1966). Clearly some unfavourable factor(s) are operating at Wytham and cause Cyzenis to be less effective than in Canada. This paper is concerned with the mortality factors acting upon the Cyzenis population, and a subsequent paper will deal with the construction of a population model describing the fluctuations in parasite density observed in the field.