Intensive management practices and pollution can influence the soil microbial biomass. Changes in the size and activity of the biomass can affect the C mineralization, turnover of organic matter and the cycling of N and P and their availability to plants because the biomass is a dynamic pool containing appreciable reserves of these elements. The microbial biomass itself can be an important indicator of soil quality, and the ratio respiration-to-biomass carbon, called specific respiration activity or metabolic quotient (qCO₂), is an important indicator of the efficiency of substrate use and it can be used as an indicator of environmental stress since it is calculated from parameters which are very sensitive to environmental changes. The aim of the present work was to study the effects of the crop and the agricultural practices on metabolic quotient in tropical soils located in the Valencia Lake watershed (Venezuela). Ten soils were selected, five of them from a lacustrine origin, and the other five from an alluvial origin. Two of the selected soils support natural vegetation, they have never been irrigated and have undergone minimal anthropogenic disturbance (control soils).The others soils are cultivated with sugarcane or banana and irrigated with waters of different origin (waste-, ground- and lake-water). The lacustrine soils are classified as Mollic Ustifluvents, loamy, carbonatic, mixed, isohyperthermic, and the alluvial soils are classified as Fluventic Ustropepts, coarse-loamy, mixed, isohyperthermic, except one of them, which is a Fluventic Haplustolls. In each site, twenty soil samples were collected randomly at a depth of 0-5 cm using a core. Basal respiration was determined by measuring the evolved carbon dioxide in a 48 h incubation experiments at 22 °C and microbial biomass carbon was estimated by Substrate Induced respiration method. The cultivated soils had a content of total organic C ranged from 0.89 to 5.40 %. In the control soils the percentage of total organic C was between 4.75 and 8.14 %. Soil pH varied from 6.96 to 7.91. The metabolic quotient ranged from 1.8 x 10^-3 to 4.8 x 10^-3 µg C-CO₂ µg^-1 microbial biomass C.h^-1. These values were similar to those previously reported for natural or agricultural soils. In the lacustrine and alluvial soils, the highest values of metabolic quotient were observed in the soils cultivated with banana, which have been treated with farmyard manure since about 6 to 20 years. Moreover, the alluvial soil under banana irrigated with untreated waste water showed significant higher metabolic quotient than those irrigated with treated waste water. In the lacustrine soils cultivated with sugar cane, the metabolic quotient was higher in the soil irrigated with a mix of industrial and domestic waste waters (whose Cd, Fe and Mn concentrations exceeded the maximum level established by Venezuelan legislation) than in the soils irrigated with underground waters or lake waters. There were no significant differences between these two last soils. Neither there were significant differences among the metabolic quotient evaluated in alluvial soils covered by sugar cane, irrigated with treated or untreated waste water. The lacustrine control soil, which has higher electric conductivity (11 dS m^-1), showed the highest metabolic quotient among the undisturbed soils, and this metabolic quotient was not significant different from those found in the lacustrine soil under banana. A significant negative correlation was found between the microbial biomass carbon/soil total carbon ratio and the metabolic quotient (r = – 0.6876; p = 0.0000), which indicates that the smaller the microbial biomass in the soils the more active the microbial population. It could be concluded that, in the investigated soils, the metabolic quotient reflected the greater demand for energy by microorganisms in soils with incorporation of organic wastes or toxic pollutants.