Ascogregarina taiwanensis (Lien and Levine), a protist gut parasite of Aedes albopictus (Skuse), is not known to complete its life cycle within the potential competitor species, Ochlerotatus japonicus (Theobald). In a laboratory cross infection study we demonstrated that A. taiwanensis completed its life cycle within Oc. japonicus and remained infectious. We also sampled cohabitating mosquito larvae in Mercer County, NJ, and based on ribosomal DNA sequence data, we determined that Oc. japonicus cohabitating with Ae. albopictus can become infected with A. taiwanensis.