| Summary: | The terrestrial land snail <i>Theba pisana</i> is circum-Mediterranean in native range and widely introduced and pestiferous in regions around the world. In California, USA, <i>T. pisana</i> has been recorded intermittently since 1914, but its source population(s) are unknown, and no morphological or molecular analyses within or between California populations have been published. Therefore, we compared molecular data (CO1, 16S, ITS2) and internal morphology (jaw, radula, reproductive system) in <i>T. pisana</i> collected from Los Angeles and San Diego counties in 2019–2020. DNA barcode (CO1 mtDNA) analysis revealed that <i>T. pisana</i> from Los Angeles County was most similar to <i>T. pisana</i> from the Mediterranean island of Malta, and northern San Diego County-collected specimens were most similar to <i>T. pisana</i> from Morocco. Morphology of the jaw and mucous glands also differed between Los Angeles and San Diego populations, but it is unclear if traits are lineage-specific or artifacts of ontogeny. Several pathways of introduction into Southern California are possible for this species, but evidence for intentional vs. accidental introduction of present populations is lacking. Subsequent investigation(s) could use the data generated herein to assess the provenance of <i>T. pisana</i> elsewhere in California and/or worldwide and inform analyses of reproductive biology and systematics in this widespread species.
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