The present investigation entitled, Field efficacy of selected insecticides against aphids infesting tomato (Lycopersicon esculentum Mill.) in district Namakkal (Tamil Nadu) was carried out at agricultural land at Mallasamudram, Namakkal district (Tamil nadu) in year 2020-2021 during rabi season. The field laid in RBD with seven treatements and one controlled plot. On the basis of the effectiveness of different treatments against tomato aphids all the insecticidal treatments were significantly effective against aphids over untreated control. The treatment with acetamiprid 20SP recorded lowest (1.79 aphids/3 leaves/plant) aphid population and emerged as superior treatment. Next effective treatment was fipronil 5%SC (1.81 aphids/3 leaves/plant) followed by imidacloprid 17.8%SL @ 125 ml/ha (1.91 aphids/3 leaves/plant), flonicamid 50%WG (1.97 aphids/3 leaves/plant), cypermethrin 25EC (2.44 aphids/3 leaves/plant, spinosad 45%SC@ 125ml/ha (2.61 aphids per 3 leaves per plant). The least effective treatment was thiamethoxam 25WG (2.65 aphids/3 leaves/plant). Cost benefit ratio was worked out, the best and most economical treatment was imidacloprid 17.8SL (1:11.37) followed by acetamiprid 20SP (1:8.41), spinosad 45SC (1:8.30), fipronil 5%SC (1:8.24), thiamethoxam 25WG (1:7.95), flonicamid 50WG (1:7.27), and cypermethrin 25EC (1:6.50), control (1:5.70).