It was the aim of this project to obtain information on the prevalence of Chlamydiaceae and Mollicutes and their potential importance for reproductive problems in cattle. Cervical or vaginal swabs were taken from 644 animals in 196 farms and blood samples were collected from 375 cattle. Out of the animals, 6.8% had aborted within the last 12 months, 2.6% showed clinical vaginitis and 11.6% clinical endometritis. Chlamydiaceae were detected and identified by PCR followed by restriction fragment length polymorphism (RFLP) analysis. For the detection and identification of Mollicutes cultivation procedures, biochemical differentiation and serological identification were used. Sera were tested for antibodies against Chlamydiaceae and Mycoplasma (M.) bovis by ELISA and against M. bovigenitalium by Western blot analysis. Chlamydophila (Cp.) abortus was found in three cervical swabs. Cp. pecorum was detected in 9% of cervical or vaginal swabs. The majority of Cp. species found was Cp. pecorum and thus fertility problems caused by Cp. abortus are limited. M. bovis was found in only one genital swab. M. bovigenitalium was rarely diagnosed (3% of cervical and 2% of vaginal swabs). M. bovigenitalium was found more often in cattle having aborted (4/32 animals) than in cattle without history of abortion (5/220, p<0.05). Ureaplasma (U.) diversum existed in 12% of cervical and 36% of vaginal swabs and was found in 8 out of 17 animals with vaginitis. Out of the animals tested, 44.9% were seropositive for Chlamydiaceae, 14.8% for M. bovis and 27.3% for M. bovigenitalium.