็High infectivity of entomopathogenic fungi to ticks under laboratory conditions has beendemonstrated in many studies. However, the few reports on their use under field conditionsdemonstrate large variations in their success, often with no clear explanation. The presentstudy evaluated the factors affecting the efficacy of the fungus Metarhizium brunneumagainst the tick Rhipicephalus (Boophilus) annulatus. It demonstrates how environmentalconditions and ground cover affect the efficiency of the fungus under field conditions. Dur-ing the summer, 93% of tick females exposed to fungus-contaminated ground died within 1week, whereas during the winter, only 62.2% died within 6 weeks. Nevertheless, the hatch-ability of their eggs was only 6.1% during the summer and 0.0% during winter. Coveringthe ground with grass, leaves or gravel improved fungal performance. Aside from killingfemale ticks, the fungus had a substantial effect on tick fecundity. Fungal infection reducedthe proportion of female ticks laying full-size egg masses by up to 91%, and reduced egghatchability by up to 100%. To reduce the negative effect of outdoor factors on fungal activ-ity, its conidia were mixed with different oils (olive, canola, mineral or paraffin at 10%v/v) and evaluated in both laboratory and field tests for efficacy. All tested oils withoutconidia sprayed on the sand did not influence tick survival or weight of the laid eggs butsignificantly reduced egghatchability. Conidia in water with canola or mineral oil spread onagarose and incubated for 18 h showed 57% and 0% germination, respectively. Comparing,under laboratory conditions, the effects of adding each of the four oils to conidia in wateron ticks demonstrated no effect on female mortality or weight of the laid egg mass, butthe percentage of hatched eggs was reduced. In outdoor trials, female ticks placed on theground sprayed with conidia in water yielded an average of 175 larvae per female and therewas no hatching of eggs laid by females placed on ground sprayed with conidia in waterwith canola or mineral oils.