1. Mosquito samples from the entire life span including senescence were analysed for weight, protein, RNA and DNA. 2. The results for each component indicate that maximal values are attained during larval development. No changes were observed during metamorphosis or the adult period. 3. The peak concentration of RNA and DNA was reached on the third day larval age. Protein content and weight were highest on the sixth day. 4. Except for the first 2 days of larval life the DNA/protein ratio was constant. Hence protein content is an index of cell number for this organism. 5. The distribution of the different biochemical components was determined in different body regions and in subcellular fractions. A high concentration of DNA was found in the soluble fraction of larval but not adult homogenates.