ABSTRACT. – Information available on the utilization of algae and cyanobacteria, with manifold potential applications in agriculture and biotechnology, for commercial biopesticide production is still scarce. We used the algal/cyanobacterial extract-larval contact method to assay in vitro against different developmental life cycle stages (the 2nd and 4th instars) of the cotton leafworm Spodoptera littoralis crude ethanolic extracts of five different algal and cyanobacterial species: Chara vulgaris (stonewort), Parachlorella kessleri, Ulva intestinalis, Cladophora glomerata (green algae), and the heterocytous cyanobacterium Nostoc carneum. All the algal and cyanobacterial treatments showed insecticidal activities against larvae of S. littoralis, with 2^nd larval instars being more susceptible. In general, P. kessleri and N. carneum exhibited the highest insecticidal toxic effects while the stonewort C. vulgaris showed the lowest insecticidal activity. Effects of these algal and cyanobacterial treatments on larval duration, % pupal formation, pupal duration and weight, % moth emergence, adults’ fecundity and longevity, eggs’ hatching, sex ratios, and larval-pupal-moth malformations were discussed. Gas chromatography-mass spectrometry (GC-MS) analyses of the crude algal and cyanobacterial extracts revealed 29 compounds, mainly containing an array of saturated and unsaturated fatty acids, some essential oils, and terpene compounds like phytol. Insecticidal activities of the algal and cyanobacterial specimens can be attributed to the synergistic effects of the abovementioned phytochemical constituents, particularly fatty acids such as the polyunsaturated ω-6 linoleic, palmitic, oleic, myristic, α-linolenic, and 7, 10-hexadecadienoic acids. The coccoid green alga P. kessleri and the cyanobacterium N. carneum can thus be recommended as ecofriendly biopesticides for the integrated pest management of the cotton leafworm and other agricultural pests.