The use of Early Life Stage (ELS) tests is a useful tool in risk assessment. The purpose of this study was to compare the sensitivity of the seabream (Sparus aurata) larvae with the extensively used Microtox test on a commercial formulation containing simazine, an s-triazine herbicide. To this end, survival, growth and histopathological changes displayed by seabream yolk sac larvae exposed during 72 h post-hatching to nominal concentrations of the commercial preparation up to its saturating concentration in water, and bioluminescence of the marine bacteria Vibrio fischeri (Microtox) were studied. Survival of larvae after three days of exposure was significantly reduced in the highest (4.5 mg/l) concentration, but no effects on growth were found in any of the simazine treatments. The 72 h LC50 value for yolk sac larvae was estimated as 4.19 mg/l. Commercial grade simazine did not exert any significant toxicity to the marine bacterium V. fischeri at the concentrations tested.