This study shows that streptomycin has both the non-specific effect, short of toxicity, of disrupting developmental sequences in larval anurans and the phase specific effect of altering tail fin regeneration. Streptomycin seems to inhibit regeneration of mesenchymal elements while leaving ectodermal components able to proliferate though in quantitatively different amounts. It appears, therefore, that the regressive, histolytic stages of regeneration are not significantly affected by streptomycin, but the phases of cytoredifferentiation of muscle and connective tissue amounts and the regulation of somite pattern are progressively curtailed with exposure to the drug.