Microbial methylation of arsenic impacts both the toxicity and fate of this environmental contaminant, and is an important component of its biogeochemical cycle. This transformation occurs in flooded paddy fields where soil microorganisms can produce dimethylated arsenic, which causes the straighthead disease in rice. The responsible anaerobic microorganisms have remained elusive because their isolation is laborious, especially as the active methylators cannot be rapidly screened. Here, we introduce a novel approach to specifically target these microorganisms. This approach is based on a high-throughput isolation technique involving microfluidic encapsulation, fluorescence-activated cell sorting (FACS), and biosensor-aided screening of microbial function. Using this method, we isolated two arsenic-methylating anaerobes from a paddy soil. This approach has the potential to rapidly obtain novel isolates. For instance, we show that one isolate actively methylates arsenate (AsV), a previously unknown phenotype in anaerobes.