Field experiments were conducted during rabi 2002-03 and 2004-05 at the Regional Agricultural Research Station, Lam, Guntur, Andhra Pradesh to study the optimum time and dose of post-emergence herbicides for Echinochloa spp. control in blackgram grown as relay crop. Results indicated that all the post-emergence herbicides like fenoxaprop-P-ethyl, clodinafop-propargyl and cyhalofop-butyl significantly reduced Echinochloa colona growth and increased blackgram yield by 27 to 42% over weedy check without any crop injury. Among different herbicides and their doses, fenoxaprop 68 g/ha recorded the highest seed yield (1332 kg/ha) and net monetary returns (Rs. 21,993/ha) and B : C ratio of 1.95 and was on par with its lower dose (56 g/ha). Herbicide application at 21 and 28 days after sowing (DAS) resulted in significantly higher reduction in Echinochloa colona density at 20 and 40 days after treatment (DAT) and total weed dry weight compared to its application at 14 DAS. However, blackgram seed yield did not differ significantly due to the time of herbicide application