Methylparaben (MP), a widely used preservative, is increasingly recognized as an environmental contaminant of concern due to its persistence in drinking water (DW) sources [1]. This study explores the interaction of MP with dual-species biofilms composed of Acinetobacter calcoaceticus and Stenotrophomonas maltophilia, formed on polypropylene (PPL) surfaces using a multiple-cylinder biofilm reactor (MCBR). Biofilms were cultivated under conditions mimicking DW distribution systems for 7 days and continuously exposed to MP at an environmentally relevant concentration (15 µg/L) [2,3]. MP exposure led to a 1-log-fold increase in culturable cell counts, indicating potential utilization of MP as a carbon source to enhance biofilm development. The presence of this environmental contaminant also resulted in a reduction in biofilm water content by 10%, leading to greater compactness and alterations in the composition of extracellular polymeric substances (EPSs), with polysaccharide and protein content decreasing by 44% and 55%, respectively. These changes were associated with heightened biofilm tolerance to chlorine disinfection. MP-exposed dual-species biofilms demonstrated lower log reductions (nearly half) in culturability after free chlorine treatment in comparison to non-exposed counterparts. Additionally, MP presence increased the release of non-damaged bacterial cells in 1.33 log cells/mL and viable but non-culturable cells (VBNCs) (3 log cells/mL increase) into the bulk phase during disinfection, raising concerns about microbial persistence and the potential for downstream contamination. These findings shed light on the interactions between MP and biofilms, emphasizing the potential risks associated with MP contamination in DW distribution systems and its implications for biofilm management.