In the pot experiment the effects of contamination of diesel oil of proper brown soil developed from light loamy sand on microorganisms abundance and activity of soil enzymes were studied. Diesel oil was applied at the following rates: 0, 2, 4 and 6 cm3 kg-1 of the soil. Trial was performed in two experimental series: amended or not amended with straw. It was found that soil pollution with oil stimulated development of Azotobacer sp., oligotrophic, copiotrophic and ammonifiers bacteria; whereas inhibited fungi proliferation, dehydrogenases and urease activities and did not affect actinomycetes number. Diesel oil addition to the soil reduced positive effects of straw on microbiological and biochemical soil properties. Oil pollution adversely affected growth and development of triticale. Straw amendment diminished level of oil toxicity to triticale.