Diastylis hammoniae Zimmer, 1902 and D. planifrons Calman, 1912 are redescribed and illustrated on the basis of examination of the type material and additional specimens collected in Subantarctic waters off Argentina during Cruise 6 of the R/V Eltanin in 1968. Both of these diastylid species have not been reported since their original descriptions. Diastylis hammoniae, which was originally described from two specimens taken in waters off eastern Patagonia, is distinguished from other members of the genus by a combination of characters including (1) the absence of exopods on pereopods 3 and 4 in the female, the shape and minute spination of the carapace, (2) the elongate carpus of pereopod 2 (distinctly longer than combined length of propodus and dactylus), (3) the telson not extending as far as the distal third of the uropodal peduncle, and (4) the presence of 6 or 7 pairs of narrow, elongate, simple lateral setae on the telson. Specimens of Diastylis planifrons were collected in the same general area as its type locality. This species is distinguished from other species of Diastylis by a combination of characters including its uniquely sculptured carapace, a well-developed antenna, and pereopod 2 having a slender and elongate carpus which, like that of D. hammoniae, is nearly twice as long as the combined lengths of the propodus and dactylus. Diastylis hammoniae and D. planifrons co-occurred in the R/V Eltanin sample.