- Two trials were conducted to determine the effects of dietary enrichments with the microalga Parietochloris incisa, rich in arachidonic acid (ARA), on stress resistance in guppies Poecilia reticulata. The microalga was added to commercial diets as a neutral lipid (NL) extract and its fractions or as broken cells. Experimental diets were applied for a period of 14 days. In trial 1, commercial diets were supplemented with NL (containing 25 mg ARA and 0.11 mg β‐carotene g−1 feed), its triacylglycerol (TAG) fraction (containing 25 mg ARA g−1 feed and no β‐carotene) and the β‐carotene fraction (containing 0.11 mg carotenoid g−1 feed and minute amounts of ARA). Neutral lipid‐fed fish demonstrated the highest resistance (P<0.05) to osmotic stress (32‐ppt NaCl), followed by fish fed with diets supplemented with TAG and β‐carotene alone, which were more resistant than control (P<0.05). In trial 2, fish fed diets supplemented with higher levels of broken alga (26.1 mg ARA g−1 feed) were more resistant (P<0.05) to stress as compared with fish fed lower ARA (16.3 mg g g−1) or an unsupplemented control diet. We suggest a dietary supplementation with broken P. incisa cells to enhance stress resistance in guppies before a stressful event.