Recent findings show that several species of parasitic flatworms that infect the marine snail, Cerithidea californica, have a division of labor in which individuals are specialized for either reproduction (reproductives) or defense of the colony (soldiers). Until now, research has focused on understanding the morphology, distribution, and behavior of the parasites in the more common, single infections. Many studies show that these parasites will kill each other when a snail is simultaneously infected with two species, but little is known about the details of these interactions. Here, we provide insight into these interactions by examining the morphology and distribution of these organisms in snails with double infections. These double infections are a snapshot of a transition that is occurring, since two different parasite species do not share their snail host for an extended period of time. Consistent with our predictions, the subordinate species is displaced; fewer soldiers are present and the reproductives look unhealthy. The incoming, dominant species has many immature parasites because it is beginning to establish an infection. These small and seemingly insignificant creatures are important when it comes to the make up of an ecosystem; they take up resources and affect the health and survival of hosts in estuarine ecosystems. In order to evaluate how they affect a community, we need more studies about the division of labor and interactions among the different parasite species.