A biological investigation of the apparently intimate and mutually relationship exist­ing between certain plant-like micro-organisms and certain insects. The literature of the subject is briefly re­viewed and discussed and a critical appraisal of previous work is made. A problem, the association of certain micro-organisms with two species of grain feeding wee­vils, is discussed in greater detail and a method of approach is proposed and analysed. Studies of the micro-organisms in vivo and in vitro are described in conjunction with investigations, by experimental techniques, of the associa­tion of the micro-organisms and the weevils. Findings are discussed and conclusions are presented. / Thesis / Doctor of Philosophy (PhD)