This research aimed to evaluate the initial root growth of wheat and rice subjected to increasing doses of thiamethoxam, the doses were 0; 8.75; 17.5; 35 e 70 g a.i. per 100 kg of seeds for wheat and 0; 17.5; 35; 70 e 105 g a.i. per 100 kg of seeds for rice, applied in seeds and to growing in mini-rhizotrons placed at greenhouse. Daily evaluations began after the emergence of the root, for ten days. The bio-activator showed great variability in the response of daily root growth, when the effect was significant the increasing doses led to length reductions.Esse trabalho objetivou avaliar o crescimento inicial das raízes de trigo e arroz, submetidas a doses crescentes do tiametoxam. As doses do bioativador foram 0; 8,75; 17,5; 35 e 70 g i.a. por 100 kg de sementes para trigo e 0; 17,5; 35; 70 e 105 g i.a. por 100 kg de sementes para arroz, aplicadas via tratamento de sementes, com cultivo em mini-rizotrons alocados em casa-de-vegetação. Avaliações diárias iniciaram-se após a emergência da raiz, por um período de dez dias. O bioativador apresentou grande variabilidade na resposta do crescimento radicular diário, quando seu efeito foi significativo as doses crescentes levaram a reduções no comprimento. Root growth in wheat and rice seeds treated with bio-activatorThis research aimed to evaluate the initial root growth of wheat and rice subjected to increasingdoses of thiamethoxam, the doses were 0; 8.75; 17.5; 35 e 70 g a.i. per 100 kg of seeds for wheatand 0; 17.5; 35; 70 e 105 g a.i. per 100 kg of seeds for rice, applied in seeds and to growing in minirhizotronsplaced at greenhouse. Daily evaluations began after the emergence of the root, for tendays. The bio-activator showed great variability in the response of daily root growth, when the effectwas significant the increasing doses led to length reductions.