The recently discovered prevalence of Rozellomycota in aquatic ecosystems establishes this poorly understood, holomycotan lineage as a group of critical importance for understanding aquatic ecology and fungal evolution. While gradually more genomes become available, knowledge on the ecology of Rozellomycota is still very scarce. After compiling their occurrences from public databases, we can clearly establish their abundance in aquatic environments, interestingly, with a dominance in carbon-rich sediments. Similarly, the carbon and nutrient rich artificial wastewater environment may serve as an example for a highly reduced environment that is inhabited by a diverse range of Rozellomycota lineages, despite being a prokaryote dominated environment. As the Rozellomycota from anaerobic environments are not monophyletic, the questions arise how the Rozellomycota adapt to differing redox conditions and if their parasitism could also have led to anaerobiosis.