This study developed a method for the assay of respiration in free swimming Caenorhabditis elegans (C. elegans). The technique employed direct monitoring of acidification of medium by these nematodes. Samples which might affect respiration were examined, including sugars and Escherichia coli (E. coli), the laboratory food source for C. elegans. The bacteria stimulated respiration but the sugars did not. The decrease in fluid environment pH is blocked by sodium azide, a known mitochondrial inhibitor. This monitoring was achieved using pH probes.