Rust caused by Phakopsora pachyrhizi H. & P. Sydow was observed for the first time on soybean (Glycine max [L.] Merr.) in Nigeria during the 1999 main soybean growing season. The disease was first noticed on soybean farms in Oniyo and neighboring villages located about 20 km west of Ogbomoso (Long. 4° 02′ E and Lat. 8° 06′ N), Oyo State, Nigeria. The area falls under the moist savanna vegetation with a bimodal rainfall pattern (from April to July, September to November) and an annual mean rainfall of 1,370 mm. Commercial cultivation of soybean by farmers in the area started in 1986. A similar disease outbreak in 1999 on soybean in Benue state, the major area of soybean production in Nigeria, could not be confirmed to be rust. In the Oniyo area, soybean is commonly grown as an early season crop usually planted between March and April in small plots to produce seeds for the main crop, normally planted in July. Rust was not observed on the early season soybean crop grown in the area in 1999. Symptoms were first observed on leaves close to the ground in September 1999 as chlorotic spots, which progressed to reddish brown lesions, then uredia. The symptoms appeared further up the plant until all the leaves were infected. Pimple-like uredia observed in the lesions were more pronounced and numerous on the lower surface of the leaves than on the upper leaf surface. Uredia were noted to cluster together in clumps (1). The urediospores released from a central ostiole in the uredia were hyaline and oblong, 17 μm wide (15 to 18 μm) and 22 μm long (19 to 25 μm). Premature defoliation occurred on infected plants. Seed weight of TGX 1485-1D, TGX 1448-2E, and TGX 1440-1E was reduced by 28, 52, and 49%, respectively. Disease severity was higher on the medium maturing cultivars and all those that were planted late.