Radioactive tracers were used to study the C allocation to coarse and fine roots, aboveground plant tissues, mycorrhizal lipids, belowground respiration, and soil in a mycorrhizal association. Sorghum bicolor (L.) Moench was grown in soil with a nonmycorrhizal microbial inoculum with and without Glomus clarum , a mycorrhizal inoculant. Fifty‐one‐day‐old mycorrhizal (M) and nonmycorrhizal (NM) plants were subjected to a 3‐h exposure to 14 CO 2 and sequentially harvested after 52, 54, 57, 64, and 76 d. Mycorrhizal plants assimilated 21% more 14 C than NM plants, even though they were slightly smaller in size. They also had a higher percentage and absolute allocation of 14 C to root tissue, belowground respiration, and soil. Mycorrhizal roots had a higher content of total lipids and total fatty acids. The fungal fatty acid 16:1ω5, usually associated with arbuscular mycorrhizal fungi, comprised up to 29.5% of the total fatty acid content of M roots, while NM roots had only trace levels of this molecule. Thin‐layer chromatography was used to separate the fatty acids extracted from the roots. The 14 C of the various components was determined by radiography. The 14 C mean residence time (MRT) of the mycorrhizal fatty acid 16:1ω5 was calculated at 7.1 d. The monoenoic, saturated, and total fatty acids had MRTs ranging from 11.1 to 14.3 d. The lipids of NM roots incorporated less 14 C label. This underscores the difference in the lipid C cycle between the M and NM roots. Translocation of the 14 C to soil was 6.3% of the photosynthesized C in the M plants relative to only 2.4% in the NM plants, giving an indication of its movement into the mycorrhizal hyphae as well as to the soil.