The present work evaluated the effects of betaine as a feed supplement on the survival and growth of Pacific white shrimp Litopenaeus vannamei reared at extreme salinities (0.5 or 50‰). Juvenile Pacific white shrimp (mean individual weight, 0.18 g) were reared in 16 tanks: eight tanks held water at 0.5‰, and eight held water at 50‰. Shrimp were maintained for 8 weeks in four replicate tanks from each salinity treatment and offered feed with or without a betaine supplement.