Ambigolimax valentianus is an invasive European terrestrial gastropod distributed throughout California. It is a serious pest of gardens, plant nurseries, and greenhouses. We evaluated the bacterial microbiome of whole slugs to capture a more detailed picture of bacterial diversity and composition in this host. We concentrated on the influences of diet and environment on the Ambigolimax valentianus core bacterial microbiome as a starting point for obtaining valuable information to aid in future slug microbiome studies. Ambigolimax valentianus were collected from two environments (gardens or reared from eggs in a laboratory). DNA from whole slugs were extracted and next-generation 16S rRNA gene sequencing was performed. Slug microbiomes differed between environmental sources (garden- vs. lab-reared) and were influenced by a sterile diet. Lab-reared slugs fed an unsterile diet harbored greater bacterial species than garden-reared slugs. A small core microbiome was present that was shared across all slug treatments. This is consistent with our hypothesis that a core microbiome is present and will not change due to these treatments. Findings from this study will help elucidate the impacts of slug-assisted bacterial dispersal on soils and plants, while providing valuable information about the slug microbiome for potential integrated pest research applications.