ABSTRACT Background: Myzus pe.rsicae (green peach aphids) are crop pests; they are among the most destructive insect pests on cultivated plants in temperate regions. Many of them are green in color, but others may be white wooly or black (McGravin,1993).Their damage to plants has made farming a complicated venture across the whole world. From the zo9logical standpoint, they are a highly successful group of organisms (Piper, 2007). The study was conducted to evaluate .the chemical composition and pesticidal effects of wood ash from Cassia didymobotrya on aphids affecting crops. DUDU® (Acelamectine) was used as a positive control.in a concentration of lml/lL -of water. It was hypothesized that Cassia didymobotrya ash has pesticidal effects on aphids which increase with. an .increase in its concentration. The study was carried out at KIU-WC Pharmacognosy laboratory. Ash obtained from a 1 year Cassia didymobotrya tree was exposed to aphids obtained from tomato p1ants i11 varying concentrations and also subjected to qualitative analysis. Results: From the results of the qualitative analysis of this ash, it contains chlorides, phosphates, sulphatcs and Calcium ions and all except Calcium are suspected causes of the death of the aphids. This is because, );:- Chlorides,· chloride ions are used in dichlorodiphenyltrichloroethane (DDT) a notorious pesticide which because of its toxicity from its organic component is being withdrawn from market (Lear and Linda, 2009). · ~ Sulphates; inorganic Sulphur (contained in 'sulphates) is a generally accepted pestici9c that was used in earlier times to kilr pests (Michael et al. , 2002 ). ~ Phosphates: these are the main ingredients used from both natural and synthetic pesticides. It was also discovered that th ash from Cassia didymobottya is effective in killing Myzus persicac and that its efficacy at a concentration of 5mg in 30Ml of water is 98.64% while the efficacy at a concentration of lOmg in 30 Ml of water is 99.04%. Conclusion: Wood ash from Cassia didymobotrya ontains chlorides, sulphates, phosphorous and calcium ions because of these chemicals the acacia wood ash is effective against aphids. The efficacy of this ash is dependent on the concentration of the ash; that is, the higher the concentration of the ash, the higher its ·efficacy, except for forming a paste at higher concentrations that would make its application difficult by blocking the pump nozzle. There is need for more research in isolating the effective component of this ash that specifically causes death of the aphid.