Abstract: We investigated the Effects of plantation development,seasons,and soil depth on soil microbial indices in Gmelina arborea plantations in south-western Nigeria.Soil samples were obtained from the soil depths of 0-15 and 15-30 cm from plantations of six different ages during the rainy season,dry seasons,and their transitions.We used plate count and fumigation-extraction methods to determine microbe population and microbial biomass carbon (MB-C) and nitrogen (MB-N),respectively.Plantation age did not affect microbial indices,implying a non-significant effect of plantation development on microbial communities.It could also imply that soil microbial indices had already stabilized in the sampled plantations.Seasonal variation and soil depth had significant effects on microbial indices.At 0-15 cm soil depth,mean MB-C increased from 50.74 μg·g-1 during the peak of the dry season (i.e.March) to 99.58 μg·g-1 during the peak of the rainy season (i.e.September),while it increased from 36.22 μg·g-1 to 75.31 μg·g-1 at 15-30 cm soil depth between the same seasonal periods.Bacteria populations and MB-N showed similar increasing trends.Correlations.between MB-C,MB-N,microbe populations,and rainfall were positive and linear.Significantly higher microbial activities took place in the plantations during the rainy season,increased with soil wetness,and decreased at greater soil depth.