The mud aqueous fraction (MAF) and suspended particulate phase (SPP) of low-density lignosulfonate type mud with úerrochrome added were nontoxic to larvae during the complete larval development of Rhithropanopeus harrisii. Five percent (5000 ppm, 0.5 % v/v mud in water) MAF and SPP were not toxic to Callinectes sapidus. Survival of C. sapidus larvae decreased as concentrations of MAF and SPP increased from 5 % (5000 ppm, 0.5 v/v mud in water) to 50% (50000 ppm, 5 % v/v mud in water). No larvae reached the 1st crab stage in 100% (100000 ppm, 10% v/v mud in water) MAF and SPP. Statistical analysis of the data on survival, mortality, and behavior are presented. Blue crab larval behavior is affected by exposure to MAF and SPP with the general effect being a decline in swimming speed. A significant reduction was only observed in 100% MAF but was noticed in 5, 25, 50, and 100% SPP. © 1984 D. Reidel Publishing Company.