The investigation presents the assessment of species composition and structure of microbiota communities in the Otap Head Cave. Species were identified using standard approaches and cultivation methods. The abundance of algae and cyanobacteria was estimated applying the 5-point Brown-Blank scale. Biodiversity of biofouling communities was revealed. Cyanobacteria were the dominant group of phototrophs colonizing the cave wall and water streams. The most frequently documented cyanobacteria were species from genera Chroococcus, Gloeocapsa, Oscillatoria, and Phormidium. Among micromycetes prevailed Ascomycetes (genera Aspergillus, Penicillum, Trichoderma). The development of so-called lampenflora around artificial lights was not observed. The presence of sulphate-reducing and sulphur-oxidizing bacteria was detected, which possibly indicates that a small circulation of sulphur occurs in the cave at present time. © 2019, Kaunas University of Technology. All rights reserved.