Apparent parasitism and the population trend during the larval stage of the host are the basic data for assessing budworm larval mortality caused by parasites. Apparent parasitism is estimated from samples of the host population, and three methods of obtaining this estimate within predetermined error limits are outlined. The seasonal history of the parasite is divided into a 'pre-emergence' period, when the parasite is developing within the host, and an 'emergence' period, when the host is actually killed. Percentage mortality is then based on host density at the beginning of the emergence period so the results will not be influenced by the mutual interference of other factors during the pre-emergence period. One field example illustrates the technique and its application to life tables.