The present study aimed to evaluate the toxicological as well as biochemical effects of the three photosensitizer compounds; rose bengal, eosin yellow lactone and methylene blue on the fourth larval instar of the cotton leaf worm, Spodoptera littoralis (Boisd.). Results of insecticidal activity against the fourth larval instar  of S. littoralis revealed that rose bengal is the most toxic photosensitizer compound followed by eosin yellow lactone, then methylene blue. The corresponding LC₅₀ values were 37X10^-5M, 81X10^-5M and 136X10^-5M; respectively. Concerning, the photodynamic process of the thee photosensitizer compounds to control the 4^th instar larvae of S. littoralis , the LT₅₀ values of the lowest concentrations of rose bengal, eosin yellow lactone and methylene blue were >4hrs for each. On the other hand, the LT₅₀ values of the highest concentrations were 2.2, 3.25 and 2.50 hrs; respectively. This indicates that rose bengal was the most active compound.  On the light of the median lethal effects of the three tested photosensitizer compounds on the total protein ,total lipids and total carbohydrates content in the tested fourth larval instar of the cotton leaf worm, the results proved that the three photosensitizer compounds decreased these biochemical contents except methylene blue slighty increased total protein content relative to control.