Inoculation of maize (Zea mays L.) roots with the mycorrhizal fungus Glomus, intraradices resulted in an increase of indole-3-butyric acid (IBA) during early stages of infection compared to control roots. The increase in IBA was accompanied by an increase of IBA synthetase activity, but the enzyme activity was also enhanced at later stages of infection. No IBA was detected in spores of Glomus, whereas small amounts of indole-3-acetic acid (IAA) were found. The endogenous IBA concentration is not important for colonization of roots with the fungus, since two other maize varieties with lower IBA content had the same infection rate as the variety with higher IBA content. The increase of IBA in AM-colonized roots was also confirmed in the variety 'Alize'.