The first aim of this study was to determine the effect of different agricultural management practices used in broccoli crop (ecological, integrated, conventional and the latter management system with addition of pig slurry) on the microbial activity of a semi-arid soil. The second purpose of the present study was to determine the effect both of the broccoli plant presence (crop and fallow) and the soil conditions (temperature and moisture) on the soil microbial activity of the above mentioned agrosystems. Soil samples were collected from the four different agrosystems in two different periods of the broccoli crop: One in March 2004 when the broccoli plant were growing and other in September 2004 without broccoli plant (after a fallow period of four months). Microbial biomass C, microbial activity parameters as basal respiration, dehydrogenase activity and ATP, and more specific parameters such as the hydrolases activity of urease, protease-BAA, alkaline fosfatase and ß-glucosidase were measured in the different soil samples. Moreover two soil organic C fractions (water soluble organic C and water soluble carbohydrates) which are readily available as energetic substrates by the soil microorganisms were measured. Results showed significantly differences (p<0.05) between the most part of parameters related with soil microbial activity in the different agricultural management systems studied, during the broccoli growth period. Large variations of the microbial activity were detected between the soil samples collected during the broccoli crop and those collected after a fallow period. Thus a factorial analysis of data detected a different effect of the agricultural management systems on the soil microbial activity when the samples were collected during the broccoli crop or after a fallow period. Also, the factorial analysis indicated that ATP, microbial biomass C, diphenol oxidase activity, urease activity and water soluble carbohydrates had a high weight in the Factor 1 which explained a 44 % of the data variance.