The aim of this experiment was to study the interactions between soil acidity and the source of soil microorganisms in determining nitrification rates of soils. It was hypothesized that (i) nitrification rates of soils decline as the soil pH decreases whereas the mineralization of organic N is unaffected (ii) the population of ammonia oxidising bacteria in acid soils is more tolerant to acidity than this in neutral soils. Two soils, a slightly alkaline soil from Israel (Typic chromoxerert) and an acidic soil from Kenya (Petroferric haplustox) were used in this study. An experiment to test the above hypotheses was designed, which included the following treatments: Sterilized soil, acidic or alkaline, with extract from the same soil; Sterilized soil, acidic or alkaline, with extract from the opposite soil; Sterilized soil, alkaline soil acidified to pH 5.0 or acidic soil limed to pH 7.5, with extract from the same or the opposite soil; Non sterilized alkaline or acidic soil. The soils were incubated for 3, 14, 28, 42 and 84 days at optimum moisture (80% WHC) and 30oC. At the end of each incubation period, the soil solutions were analyzed for NH4+, NO3-, pH and electro conductivity as well as the soil potential nitrifying capacity (potential ammonium oxidation). As expected, the rate of nitrification was slower in low pH than in alkaline or neutral pH. The source of the microorganisms' population influenced the nitrification rate, namely, the nitrification rate under low pH with microorganisms from the acid soil was faster than with microorganisms from the alkaline soil. However, the dominant effect was the soil pH. The minerlization rate was also slower under low pH. The soil potential nitrifying capacity was lower under acid conditions especially with microorganisms from the alkaline soil. The ecological significance and agricultural implications of these findings will be discussed.