The degradation of chlorophyll a (Chl a), carotenoids and phycobiliproteins, and the fluorescence responses of the filamentous cyanobacterium Anabaena doliolum Bhar. were studied with short exposure (45 min-30 hours) to the pyrethroid insecticide cypermethrin by taking 20 µM and 50 µM of the chemical as treatment concentrations. There was significant reduction in Chl a, carotenoids and phycobiliprotein contents of cells in each of the selected concentrations of the insecticide. Pheophytin a and fluorescing and non-fluorescing chlorophyll catabolites (FCCs and NCCs) were produced as degradation products of Chl a during the exposure period. The amount of the degradation products gradually decreased with prolonged exposure of the cells to the insecticide. The pattern of degradation of carotenoids was similar to that of Chl a but the rate of degradation was less than for Chl a. The amount of lycopene also continuously decreased with increase of the insecticide concentration. There was gradual reduction of cellular phycocyanin and phycoerythrin contents of cells with rapid degradation during the first 6 hours of treatment. The fluorescence emission of phycobilisomes and photosystem II decreased with increased concentrations of insecticide and prolongation of treatment. The cyanobacterium did not show any recovery from insecticide stress during 30 hours of exposure.