For use in an in-situ hybridization method, three probes (HM, TR and BL) were designed to hybridize, respectively, with (1) Histomonas meleagridis, (2) Tetratrichomonas gallinarum, and (3) a broad range of micro-organisms, including Blastocystis spp. Mono-eukaryotic cultures were used to test the specificity of the three oligonucleotides and to optimize the hybridization procedure before applying the probes to archived samples of various tissues and to a culture of Trichomonas gallinae. Specific detection of H. meleagridis was possible with the HM probe, but the other two probes were less specific. The TR probe detected members of the Trichomonadidae (Tetr. gallinarum and Tr. gallinae). Positive signals from a great variety of microorganisms, including fungi and protozoa from different animal species, were obtained with the BL probe. However, neither H. meleagridis nor the two members of the Trichomonadidae mentioned above were detected with this probe, allowing the exclusion of these parasites. Use of the three probes makes possible the accurate detection of H. meleagridis and its distinction from other micro-organisms in tissue samples.