Premise of research. Light has profound effects on plant development, and the interaction between red and blue light pathways can play a significant role in the phototropism of plant seedlings. The aim of this research was to clarify the involvement of red light effects on blue light phototropism in both roots and hypocotyls of Arabidopsis thaliana seedlings. Pivotal results. In contrast to the well-documented effects in hypocotyls, we found that red light inhibits blue light–based phototropism in roots of the Landsberg ecotype and that this inhibition is reduced in mutants lacking phytochromes A and B. Attenuation of blue light root phototropism by red illumination was also observed in Arabidopsis seedlings of the C24 ecotype, and this inhibition was not observed in a transgenic strain lacking all phytochromes (in which the deficiency is specific to the root only). However, in contrast to the Landsberg and C24 ecotypes, roots of Arabidopsis seedlings of the Columbia ecotype display a significant enhancement of blue light phototropism by red light pretreatment. Conclusions.Our results suggest that differences exist in the mechanisms of phototropism based on ecotype and that phytochromes are involved in the red light attenuation of blue light–based root phototropism when it occurs. This study is one of the first to consider red light effects specifically on root phototropism.