When host and parasitoid phenologies are unknown the estimation of the generational percentage parasitism poses several methodological problems. We developed a simple method, based on a statistical approach, to solve that problem. An unbiased estimator was found, named μCR, of the generational percentage parasitism, and its performance was tested through the analysis of data from two host-parasitoid systems: Phyllonorycter crataegella (Clemens) (Lepidoptera: Gracillariidae) and Sympiesis marylandensis (Girault) (Hymenoptera: Eulophidae), and Pieris rapae L. (Lepidoptera: Pieridae) and Cotesia (Apanteles) glomerata (L) (Hymenoptera: Braconidae). The μCR estimator proved to be a good approximation of the generational percentage parasitism, but its 95% confidence interval was very wide. The performance of the μCR estimator was improved and the 95% confidence interval was smaller when some information on host and parasitoid phenologies was added. The results suggested that the smaller interval μCR ± one standard deviation would contain with a great probability the generational percentage parasitism. none