SUMMARYExperiments have been carried out on carbon dioxide production from sand cultures with peptone and glucose solution inoculated with various types of bacteria and protozoa, and with glucose and ammonium sulphate solutions of varying concentrations and C/N ratio. The following results were obtained:1The presence of protozoa increases the carbon dioxide production especially in mixed bacterial cultsres2The increase of carbon dioxide production is greater in glucose solution than in peptone.3A further increase in the number of protozoa has an unfavourable effect on the carbon dioxide production.4The number of bacteria is smaller in the presence of protozoa than in their absence, but the bacterial efficiency is greater and more uniform.5The bacterial numbers and carbon dioxide production are definitely correlated in peptone, but in glucose to a less degree especially in the presence of protozoa.6The reduction of concentrations of glucose from 0–6 to 0–2 per cent. resulted in a greater percentage production of carbon dioxide.7With a lower concentration (0–2 per cent.) of glucose the presence of protozoa causes a greater increase in carbon dioxide production than in higher concentrations (0–6 per cent.).8In the absence of protozoa increasing the C/N ratio had no or only a slight depressing effect on carbon dioxide production.9In the presence of protozoa increasing the C/N ratio is followed by a marked increase in carbon dioxide production.10In bacterial cultures a lessening of the C/N ratio below 10/1 results in a fluctuation of bacterial numbers.I am indebted to Sir John Russell for his kindness in giving me facilities to carry out the work in this Institution. The work was done in Mr D. Ward Cutler's department, and I take this opportunity of expressing my gratitude for his ever ready help and unfailing kindness. For the culture ofColpidiumI am greatly indebted to Miss Jane Meiklejohn of the same dep