The acute aquatic toxicity of a new Corexit series dispersant, Corexit 9500, was evaluated and compared with that of others in the series using early life stages of two common nearshore marine organisms: the red abalone (Haliotis rufescens) and a kelp forest mysid (Holmesimysis costata). Spiked-concentration testing was performed under closed, flowthrough conditions, with dispersant concentrations measured in real time using UV spectrophotometry. Median-effect concentrations ranged from 12.8 to 19.7 initial ppm for Haliotis and from 158.0 to 245.4 initial ppm for Holmesimysis. The difference in sensitivity of the two types of tests was consistent with patterns seen with other oil dispersants. Also, these data indicate Corexit 9500 to be of similar toxicity to Corexit 9527 and 9554. Corexit 9500 represents a reformulation of a long-time industry "standard," Corexit 9527, to allow use on higher viscosity oils and emulsions. The present data suggest that acute aquatic toxicity concerns surrounding the use of this newer dispersant should not be significantly different from those associated with the use of Corexit 9527.