The evolution of nucleic acids in larvae of P. serratus reared in the laboratory from hatching to metamorphosis has been studied. From the 1st larval stage to the post-larval stage, DNA and RNA concn decrease: from 185 to 95 uMoles-bases/g PSD for DNA and from 133.5 to 78.75 uMoles-bases for RNA. The concn decrease is not regular; it is emphasized between stages 4 and 5 on the one hand, and between stage 7 and the post-larva on the other hand. The amount of DNA and RNA per larva increases proportionally to the wet weight from stage 1 to the post-larva. As the weight is multiplied by 7.6, total DNA and total RNA are multiplied by 3.80 and 4.25 respectively. The RNA/DNA ratio has a mean value of 0.77; it increases slightly from stage 1 (0.72) to the post-larva. (0.88). Growth is mostly dependent on cellular and from stage 5 to stage 7. During the transitory periods (from st.4 to st.5 and from st.7 to PL) increase of cellularsize (hypertrophy) intervenes as much as 100% in the weight increase observed. The biochemical variations noticed through the complete larval development of P. serratus seem to agree with some distinct morphological phases.