The complete sequence (14,751 bp) of the mitochondrial DNA (mtDNA) of the opisthobranch gastropod Placida sp. was determined using long PCR and genome-walking techniques. The genome contains 13 protein-coding genes (PCG), 2 ribosomal RNA (rRNA), 22 transfer RNA (tRNA) genes that are typical of metazoan mtDNA, and 2 lengthy noncoding regions with a total length of 1441 bp. All the tRNA genes have general secondary structures without reduced T or D stems. The mitochondrial gene arrangement of Placida sp. is almost identical to that of sea slugs, whereas the tRNA gene arrangement is different from other animals outside Sacoglossa in the Euthyneura. A phylogenetic analysis based on the mitochondrial genome of Euthyneura indicated that Placida sp. and Placida dendritica are closely related to Elysia chlorotica as a sister taxon, which strongly supports their close phylogenetic affinity.