We are investigating the pathway of formaldehyde assimilation in the purple non-sulphur bacterium Rhodobacter sphaeroides. Purple non-sulphur bacteria are well known for their metabolic diversity, using a wide variety of substrates for carbon and energy needs. The metabolism of some of these substrates, specifically methylated compounds, generates formaldehyde that must be further metabolized or would prove toxic to the cells. Our data suggest that formaldehyde's fate in R. sphaerodies is dependent on whether cells are grown in the presence or absence of oxygen. Under anaerobic (photosynthetic) growth conditions, R. sphaeroides is able to grow using methanol as a sole carbon source. Since formaldehyde is the immediate product of methanol oxidation, it must provide all the carbon skeletons for R. sphaeroides under photosynthetic growth conditions. Of the pathways that have previously been described for formaldehyde assimilation in microbes, enzyme assays in crude extracts suggest that R. sphaerodies may use either the Serine-Isocitrate cycle or the Calvin cycle for photosynthetic methylotrophic growth. Under aerobic growth conditions, similar enzyme assays suggest that formaldehyde is oxidized to carbon dioxide and released to the atmosphere thus detoxifiying formaldehyde form the cells. By using 13C-labeled substrates in enzyme assays, we hope to confirm the path(s) of formaldehyde oxidation products in these cells. By showing that the predicted products are formed when specific formaldehyde pathway intermediates are added to R. sphaeroides extracts, we will provide direct evidence to support our tentative pathway for formaldehyde oxidation in the presence and absence of oxygen.