Background/Question/Methods Although many invasive species are recognized as potential competitors of native species, the harm to the native competitor may be difficult to observe in the vast majority of cases. Changes to the individual fitness and population density of native species in the presence of introduced competitors should be evaluated, because these changes suggest an increased risk of eventual extinction. The native slug Philomycus carolinianus is likely to compete for resources with the aggressive invasive slug Arion subfuscus in Maryland forests. In order to establish whether competition occurs between these two species, I am testing for the following criteria: the existence of competitive displacement, the sharing of limited resources (shelter and food), and a decline in the fitness of P. carolinianus in the presence of A. subfuscus. Results/Conclusions Field surveys showed that displacement between heterospecifics does not apparently occur within mixed natural populations of these species across grids of 5 x 5 m cells. A lab experiment established that low natural levels of food (fungus) can limit the fitness of each slug species, while shelter (coarse woody debris) was not limiting, at least under the moist experimental conditions used. When sharing a low-resource lab cage with either A. subfuscus or conspecifics, P. carolinianus experienced a similar decline in fitness, suggesting that exploitative resource competition was no greater between heterospecifics than between conspecifics. No evidence of heterospecific interference (competition independent of resource levels) was found. Given the limited support for the criteria of competition, A. subfuscus was not shown to be an immediate threat to the persistence of P. carolinianus.