Herbicides are important management tools in agriculture because they can be used instead of tillage for weed control, thereby conserving soil and moisture. Laboratory experiments were conducted at the department of Agronomy University of Ibadan to determine the effect of different concentrations of the Imidazolinone single herbicides (Imazaquin, Imazethapyr and Imazapyr) on the microbial activities in selected tropical soils. The selected soil samples (i.e. soil from a teak and cultivated soil) were treated separately to give concentrations of 0, 0.15, 1.5, 15 and 150 ppm; 0, 0.05, 0.5, 5 and 50 ppm; and 0, 0.25, 2.5, 25 and 250 ppm of Imazaquin, Imazethapyr and Imazapyr respectively. Changes in total mineral nitrogen production, microbial number and carbon – dioxide (CO2) production during a 28 day incubation period were determined. Changes in mineral nitrogen were determined at weekly intervals for four weeks. The herbicides were stimulatory or inhibitory to microbial activities depending on the concentration applied to soils. Their rates of degradation and microbial proliferation were also affected by the concentrations applied. Imazaquin (Scepter) was found to inhibit ammonification in both soils. Nitrification was equally inhibited in teak soil, while lower concentrations stimulated nitrification, with higher treatment levels inhibitory to nitrification in the cultivated soil. Imazethapyr had a similar effect on ammonification and nitrification at higher levels of concentration. Arsenal (Imazapyr) was however stimulatory in cultivated soil while inhibitory in teak soil. Mineral nitrogen was observed to be generally higher in teak soil than cultivated soil for all the herbicides treatments. The application of the chemicals generally depressed the total microbial load of the soils. The depression was greater during the early period of incubation (14days). Bacteria populations were affected to a higher degree than fungi. Inhibition of microbial proliferation was noticed with the herbicides application. Key words: Cultivated Soils, Teak Soil, Mineral –nitrogen changes, Nitrification, Ammonification, Inhibitory, Stimulatory.