The work aimed to evaluate the effectiveness of the fungus Metarhizium anisopliae as a biological agent for controlling the Rhipicephalus (Boophilus) annulatus cattle ticks under laboratory conditions. This research could provide an alternative biological method to the chemical acaricides. R. annulatus ticks are a significant menace to the economic stability of the Egyptian cattle industry. M. anisopliae is a commercially available entomopathogenic fungus (EPF) that infects various insects. This research investigated the biological parameters, morphology, and ultrastructure of R. annulatus engorged female ticks treated with fungus using spraying and dipping applications (10⁶ spores/ml). On the fifth day of treatment, morphological abnormalities in Metarhizium anisopliae-treated ticks were noticed, characterized by wrinkled, white patches, and darker cuticles. Furthermore, the treated ticks' eggs had a gloomy, lifeless appearance.  By the tenth day of treatment, the mortality rate had reached 100% in the ticks in both applications. Compared to the control group, the treated R. annulatus engorged ticks revealed a significant reduction in egg mass, egg counts, and hatchability. The reproductive index was notably lower in the treated groups, with results at 0.324, and 0.320 in the dipping and spraying groups, respectively, compared to the control group of 0.815. Moreover, the dipping group exhibited 60.2% inhibition in oviposition, while the sprayed group showed a 60.7% reduction. The analysis of M. anisopliae infection using a transmission electron microscope revealed fungal penetration through the three cuticle layers. There were some outgrowing hyphae, and the treated cuticle was severely and violently damaged. The present findings suggest that M. anisopliae can infect and penetrate the cuticle of R. annulatus soon after treatment, making it a viable choice for biological tick control.