A karyological investigation was carried out on the testis cells of the tidal pool sacoglossan sea slugStiliger felinusfrom New Zealand. The haploid chromosome set was characterised by 14 elements where median‐, subterminal‐, and terminal‐centromere chromosomes were present. Using flow cytometric analysis, haploid genome size of the species was found to be 2.8 pg: the highest value encountered in the subclass Opisthobranchia.Stiliger felinusdiffers from other Stiligeridae in chromosome number and frequency of cross‐shaped bivalents in spermatocytal metaphase I plates.