The population structure and habitat selection of Anisakis simplex in 35 harbor porpoises off Denmark are described. The nematodes were collected from the stomach and duodenal ampulla and were categorized as third-stage larvae, fourth-stage larvae, subadults, and adults. The porpoises harbored 8,043 specimens of A. simplex. The proportion of adults and subadults increased with infrapopulation size. The number of development stages across infrapopulations covaried significantly (Kendall's test of concordance). Concordance was higher in hosts with the highest intensities than in those with low and medium intensities. All stages occurred mainly in the forestomach, but this trend was stronger for the adults. Adult and subadult sex ratios did not depart significantly from 1:1. Our data suggested that recruitment and duration of each stage were the main factors accounting for infrapopulation structure. The preference of A. simplex for the forestomach conformed with previous studies, but the narrower distribution of adults relative to other stages might indicate a strategy to enhance mating opportunities. Information on sex ratios of A. simplex is scarce and contradictory. We suggest that the discrepancies might partly reflect differences in categorization criteria and statistical methods.The occurrence of third-stage larvae (L3) of anisakid nematodes in the fillets of commercial fish entails well-known public health concerns and economic repercussions (Burt, 1994;Smith, 1999). Consequently, the population ecology of anisakid nematodes has received more attention than that of any other parasite of marine mammals. Most population studies of anisakids have focused on describing the population structure and dynamics of the sealworm, Pseudoterranova decipiens, and other species common in seals (see Bowen 1990; Desportes and McClelland 2001, and references therein for examples) and have produced valuable data on the life history and population ecology of these nematode species.As for the whaleworm, Anisakis simplex, most population studies have focused on its intermediate and paratenic hosts (Brattey and Bishop, 1992;Herreras et al., 2000;Podolska and Harbowy, 2003), whereas its population biology in the definitive hosts has been less studied. Most information on the definitive hosts concerns the harbor porpoise, Phocoena phocoena, a small odontocete common in the Northern Hemisphere (Smith, 1989;Brattey and Stenson, 1995;Herreras et al., 1997). The data on the population structure of A. simplex in this cetacean are based on small sample sizes (Young, 1972;Smith, 1989) or lack details of the infrapopulation structure (Lick, 1991;Brattey and Stenson, 1995). These 2 shortcomings are also common in studies of this nematode in other cetacean species (e.g., Wazura et al., 1986;Raga and Balbuena, 1993). Moreover, little is known about habitat selection by A. simplex in the definitive hosts. To our knowledge, there are only 2 studies providing such information (Brattey and Stenson, 1995;Aznar et al., 2003), and only the ...