Probes for the detection of Azospirillum strains were obtained from DNA fragments generated by random amplification of polymorphic DNA (RAPD) and tested to assess their specificity towards DNA extracted from pure cultures. The most specific probe, referred to as alpha 4, produced a hybridization signal only with amplified DNA of A. lipoferum ATCC29731. This strain was inoculated, together with two other Azospirillum strains, in soil microcosms of different complexity and its presence tested with the probe alpha 4. This probe confirmed its high specificity with amplified DNA extracted from the soil microcosm and in the presence of other A. lipoferum strains, indicating that the strategy for bacterial detection, based on RAPD markers, is useful for monitoring the presence of a particular strain under environment-like conditions. Other RAPD-derived probes, when tested on soil samples, did not show the same level of specificity as that shown on DNA from pure cultures. This result suggests that some precautions are necessary in the choice of a really specific RAPD marker. In a further development of this strategy, the alpha 4 probe was sequenced and two pairs of "nested" primers were designed, which enabled a diagnostic polymerase chain reaction from soil samples that was specific for the A. lipoferum species.