California tiger salamanders (Ambystoma californiense) exhibit an obligate metamorphic life history strategy in which aquatic larvae metamorphose into terrestrial juveniles early each summer. However, we have recently documented two occurrences of populations containing large gilled California tiger salamander larvae during the winter months. Large gilled larvae could be the result of genetic admixture between California tiger salamanders and an introduced congener, the barred tiger salamander (Ambystoma tigrinum mavortium), but the population occurrences we observed are located outside the presumed extent of the A. californiense/A. t. mavortium–hybrid swarm. We used single nucleotide polymorphisms (SNPs) to investigate the possibility the genomes of these atypical larvae have been introgressed by “superinvasive” loci that have escaped from the established hybrid swarm. We also discuss the possibility the overwintering larval phenotypes may be the result of increased prevalence of large, fishless, semipermanent ponds, or of a combination of both introgression and habitat modification.