Twenty-two derivatives of naphthalene were assayed under an acute static regime with biological activity being monitored as population growth of Tetrahymena pyriformis. Activity varied over one log unit. Substituent constant structure-activity analyses revealed the model, log BR = 0.282Ha + 0.352 pi + 0.692F + 0.3341Xvsub - 0.326R + 0.027, to be best and to account for 85% of the variation in log BR (BR, biological response; Ha, hydrogen acceptance; pi, hydrophobic substituent constant; F, polar electronic substituent constant, 1Xvsub, substituent molar connectivity index; R, resonance electronic substituent constant). The Ha and pi parameters are the most important, accounting for 71% of the log BR variability.