Gambusia zarskei is a small sized species of the subgenus Arthrophallus (SL in males usually not exceeding 25 mm and in females 35 mm), which is distinguished from all other species of the senilis species group by the following characters: shafts of distal spines of gonopodium ray 3 much prolonged, lateral projection on elbow weekly developed, vs. well developed in G. amistadensis and G. hurtadoi or absent in G. alvarezi, G. gaigei and G. senilis. G. zarskei is also distinguished by the following unique combination of characters: relative gonopodial ray 4p serra length of gonopodium long; frequency distribution of SEL/SEW radius 1.35 – 1.65, vs. 0.95 – 1.25 in G. alvarezi, 1.55 – 1.90 in G. amistadensis, 1.00 – 1.35 in G. gaigei and 0.75 – 1.05 in G. hurtadoi; elbow on gonopodium ray 4a simple and not rounded or distally hooked, vs. usually hooked in G. alvarezi, G. gaigei, G. senilis; 8 dorsal fi n rays, vs. usually 9 in G. alvarezi and G. hurtadoi; black anal spot present vs. absent in G. senilis.