A series of O-(acyl) chitosan (OAC) derivatives with a degree of substitution (DS) between 0.02 and 0.28 were synthesized by reaction of alkanoic acid derivatives with chitosan in the presence of H2SO4 as a catalyst. The reaction was performed at 80 degrees C for 4h with different mol ratios of alkanoic acids to chitosan. The synthesized compounds were analyzed by H-1- and C-13-NMR spectroscopy. A high DS was obtained with O-(butyroyl) chitosan (DS 0.28) at a mol ratio of (1: 5) chitosan to butyric acid. Their fungicidal activity against the grey mould Botrytis cinerea (Leotiales: Sclerotiniaceae) and the rice leaf blast pathogen Pyricularia grisea (Teleomorph: Magnaportha grisea) has been evaluated. O-( decanoyl) chitosan at mol ratio of 1: 2 ( chitosan to decanoic acid) was the most active compound against B. cinerea (EC50 = 1.02 g. l(-1)) and O-(hexanoyl) chitosan displayed the highest activity against P. grisea (EC50 = 1.11 g. l(-1)). It has been mentioned that some derivatives also repressed spore formation at rather high concentrations (1.0, 2.0 and 5.0 g. l(-1)). The insecticidal activity has been screened at 5 g. kg(-1) artificial diet against the larvae of the cotton leafworm Spodoptera littoralis ( Lepidoptera: Noctuidae). The results revealed that all of the synthesized derivatives showed high inhibition of growth of the larvae of S. littoralis compared to chitosan (7% growth inhibition) and the most active one was O-( decanoyl) chitosan (64% growth inhibition) after 5 days of feeding on treated artificial diet.