Many are surprised to realize that the noses of sharks contain many species of parasites Kroeyerina elongata (Copepoda: Siphonostomatoida) is a parasitic copepod commonly found packing the olfactory sacs of blue sharks (Prionace glauca). In this study, the spatial distribution of 1623 K elongata collected from the left olfactory sacs of 14 blue sharks was analyzed. Numbers of K elongate per olfactory sac ranged from 0-228 (x = 115.92 +/- 82.82), and a weak positive linear relationship (r2 = 0.213) existed between the number of copepods per olfactory sac and shark fork length. Female copepods typically outnumbered males (mean number of males per female = 0.487 0.263). Both males (96.84% 0.034) and females (99.00% 0.013) appeared positively reotactic with respect to water flow through the olfactory sac, and adults of each sex seemed to prefer different locations within the olfactory sac. Ovigerous females typically attached at the base of the rachis or within the first third of the excurrent water channel, while males tended to attach randomly about the secondary lamellae of the olfactory sac. It is interesting that the few subadult females that were collected (some of them paired with males) were distributed in male-like fashion. When these results are interpreted in light of a good understanding of the pattern of water flow throughout the olfactory sac they suggest some mechanisms by which the realized niche of K elongata might be achieved. Our hypothesis is that infective copepodids enter the olfactory sac with incurrent water as the shark swims. These larvae settle in a random fashion on the secondary olfactory lamellae and proceed through a series of molts to eventually achieve an adult male or female habitue. Adult males search the secondary lamellae for females. At the time of copulation, males and females are approximately the same size and shape. After copulation, females begin to metamorphose, via which the genital complex grows significantly. As this transformation takes place, females move to inhabit the proximal portions of the excurrent water channels between olfactory filaments. Females become ovigerous, and upon maturation, embryo sacs rupture to release free swimming nauplii larvae into the excurrent water flow of the olfactory sac. Nauplii pass out of the nose and into the surrounding water where they eventually molt into infective copepodids to complete the life cycle.