Relative quantitative RT-PCR and western blotting were used to investigate the expression of three genes with potentially regulatory functions from the arbuscular mycorrhizal fungus Glomus intraradices in symbiosis with tomato and barley. Standardisation of total RNA per sample and determination of different ratios of plant and fungal RNA in roots as colonisation proceeded were achieved by relative quantitative RT-PCR using universal (NS1/NS21) and organism-specific rRNA primers. In addition, generic primers were designed for amplification of plant or fungal beta-tubulin genes and for plant glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes as these have been suggested as useful controls in symbiotic systems. The fungal genes Ginmyc1 and Ginhb1 were expressed only in the external mycelium and not in colonised roots at both mRNA and protein levels, with the proteins detected almost exclusively in the insoluble fractions. In contrast, mRNA of Ginmyc2 was identified in both external and intraradical mycelium. In mycorrhizal roots, Ginmyc2 and fungal beta-tubulin mRNAs increased in proportion to fungal rRNA as colonisation proceeded, suggesting that accumulation reflected intraradical fungal growth. Fungal alpha-tubulin protein and beta-tubulin mRNA both appeared to be more abundantly accumulated in AM hyphae within heavily colonised roots than in external hyphae, relative to fungal rRNA. Tomato GAPDH mRNA accumulation was proportional to tomato rRNA, but accumulation of tomato beta-tubulin mRNA was reduced in colonised roots compared to non-mycorrhizal roots. These results provide novel evidence of differential spatial and temporal regulation of AM fungal genes, indicate that the expression of tubulin genes of both plant and fungus may be regulated during colonisation and validate the use of multiple 'control' genes in analysis of mycorrhizal gene expression.