Microorganisms in biological wastewater treatment plants require adaptive strategies to deal with rapidly fluctuating environmental conditions. At the population level, the filamentous bacterium Candidatus Microthrix parvicella (Ca. M. parvicella) has been found to fine-tune its gene expression for optimized substrate assimilation. Here we investigated in situ substrate assimilation by single cells of Ca. M. parvicella using nano-scale secondary-ion mass spectrometry (nanoSIMS). NanoSIMS imaging highlighted phenotypic heterogeneity among Ca. M. parvicella cells of the same filament, whereby (13)C-oleic acid and (13)C-glycerol-3-phosphate assimilation occurred in ≈21-55% of cells, despite non-assimilating cells being intact and alive. In response to alternating aerobic-anoxic regimes, (13)C-oleic acid assimilation occurred among subpopulations of Ca. M. parvicella cells (≈3-28% of cells). Furthermore, Ca. M. parvicella cells exhibited two temperature optima for (13)C-oleic acid assimilation and associated growth rates. These results suggest that phenotypic heterogeneity among Ca. M. parvicella cells allows the population to adapt rapidly to fluctuating environmental conditions facilitating its widespread occurrence in biological wastewater treatment plants.