Pythium insidiosum is the only known species of this genus capable of causing infections in humans, horses, cattle, dogs, cats and captive polar bears. We investigated the growth response of several isolates from clinical cases of pythiosis in Costa Rica (one isolate), Thailand (two isolates), and the USA (two isolates, one from Florida and the other from Tennessee), to 100 and 200 μg/mL streptomycin. It was found that one of the Thai P. insidiosum isolates was stimulated, while the other isolate from the same country was inhibited by streptomycin. The isolates from Costa Rica and Florida, USA, were not significantly affected. Considerable variation in the response of the Tennessee isolate to streptomycin was recorded in P. insidiosum cultures resulting from the transfer of 2 mm2 sections of vegetative growth, suggesting heterozygosity among its nuclei. Calcium did not reverse the inhibition of the Tennessee isolate, as previously reported in other Peronosporalean Oomycetes. Following the addition of calcium, growth enhancement, already stimulated by streptomycin, was observed in one of the Thai isolates. The finding that streptomycin may stimulate the in vitro growth of some P. insidiosum isolates indicates that the indiscriminate use of streptomycin or other antibiotics, to treat putative bacterial infections, may be deleterious to patients that may have undiagnosed pythiosis.