Two isomeric sulfur-interrupted 8-amino side chain analogues of 4-methyl-5-[m-(trifluoromethyl)phenoxy]primaquine (2) were prepared and tested for antimalarial activity. The compounds were evaluated for blood schizonticidal activity against Plasmodium berghei in mice and radical curative activity against Plasmodium cynomolgi in rhesus monkeys. In addition, they were evaluated for causal prophylactic activity against Plasmodium berghei yoelii in mice. Both compounds were more active and less toxic than primaquine in the P. berghei screen. One of the compounds showed radical curative activity similar to primaquine but was less active than 2. One of the compounds was active at 160 mg/kg in the P. berghei yoelii screen; the other was not active.