Arsenic (As) contamination in aquatic environment adversely impacts aquatic organisms. The present study assessed the toxicity of different As species and concentrations on bighead carp (Hypophthalmichthys nobilis) at early life stage, a major fish in Yangtze River, China. We measured the changes in embryo and larvae survival rate, larvae aberration, concentrations of thyroid hormone thyroxine, and transcription levels of thyroid hormone receptors (TRs) in fish larvae after exposing to arsenite (AsIII) or arsenate (AsV) at 0, 10, 30, 50, 100, or 150 mu g L-1 for 78h. As concentrations <= 150 mu g L-1 had limited effect on embryo survival rate (6-8% inhibition), but larvae survival rate decreased to 53-57% and larvae aberration rate increased to 20-24% after As exposure. Moreover, thyroxine levels elevated by 23% and 50% at 100 mu g L-1 AsIII and 150 mu g L-1 AsV. Besides, AsIII and AsV decreased the transcriptional levels of TR alpha by 72 and 53%, and TR beta by 91 and 81% at 150 mu gL(-1) As. Our data showed that AsIII and AsV had limited effect on carp embryo survival, but they were both toxic to carp larvae, with AsIII showing more effect than AsV. As concentrations <150 mu g L-1 adversely influenced the development of bighead carp larvae and disturbed their thyroid hormone homeostasis. Published by Elsevier B.V.