ABSTRACT The green water technique uses microalgae in the water of indoor larviculture, providing a darker environment to favor fish growth, welfare and health. We evaluated growth performance and locomotor activity after light exposure of pirarucu (Arapaima gigas) larvae reared in green or clear water. During one test, pirarucu larvae (3.6 ± 0.3 cm; 0.36 ± 0.1 g) were reared in 50-L circular tanks (n = 3 per treatment, 50 larvae per tank) in a static system containing green water [microalgae (w3algae; Bernaqua® 10 g m-3) added] or clear water (control). Fish weaning was achieved by co-feeding with Artemia nauplii and microdiets for seven days until full microdiet substitution. Larvae were biometrically evaluated on days 10, 17 and 24 to assess growth performance. In a second test, the locomotor activity of the larvae was analyzed before and after light exposure (1400 ± 60 lx) for 48 h according to an ethogram. After 24 days, the larvae reared in the green water were significantly heavier than those from the clear water, and displayed significantly fewer circular swimming movements. Body cortisol increased in both groups after light exposure. The microalgae provided an additional food source for larvae, with positive impact on growth until day 17 of larviculture. Green water can be a strategy to achieve better results in pirarucu larviculture, especially during and up to 10 days after the co-feeding period.