We described the population structure and habitat selection of Anisakis simplex in 46 franciscanas, Pontoporia blainvillei, 8 Burmeister's porpoises, Phocoena spinnipinis, 2 Dusky dolphins, Lagenorhynchus obscurus, and 2 common dolphins, Delphinus delphis, caught incidentally in the coastal fisheries of northern Argentina. Prevalence ranged from 50% to 100%, but mean intensities were low (1.0–3.6), suggesting that A. simplex has low recruitment rates to coastal dolphins in this area. Adult nematodes were found exclusively in the main stomach of 64.5% of franciscanas (the first stomach of this species) and the forestomach of 50% of Burmeister's porpoises and Dusky dolphins. Other developmental stages occurred in the first stomach but were also found in posterior digestive chambers. The median worm of the distribution among chambers was more anterior for preadults than for L4s and more anterior for L4s than for L3s. Our data are compatible with the hypothesis that individuals of A. simplex tend to select the site where digestion begins. This preference is more pronounced in adult worms. The greater restriction of adults to the first chamber could be viewed as a strategy to enhance mating opportunities. Alternatively, it might indicate that adults have stronger requirements for the resources provided by the first chamber.