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The degree to which invasive species have altered the demography, ranges, and microhabitat occupation of native species is poorly known. Yet, the competition-mediated decline of native populations, in concert with other factors such as habitat degradation, can place native species at risk of extirpation. Understanding whether competition between native and non-native species can take place under ordinary environmental conditions can allow us to extrapolate whether native species are likely to have experienced harm in the past and/or if they are likely to do so in the future. The native slug Philomycus carolinianus is likely to compete for resources with the aggressive non-native slug Arion subfuscus in central Maryland forests. In order to establish whether competition occurs between these two species, I tested for the following criteria: the existence of competitive displacement in the field, overlap in the use of limited resources (shelter and food), a decline in the fitness of P. carolinianus in the presence of A. subfuscus, and the action of competition mechanisms (interference and exploitation) between them. Field surveys showed that displacement between A. subfuscus and P. carolinianus does not apparently occur within mixed natural populations. Resource use of the two slugs overlapped, with part of the diet (i.e. fungus) and a large proportion of the microhabitats occupied (i.e. coarse woody debris) in common. 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. 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 criter