The interiors of 72 decaying branches from a range of tree species were investigated for the presence of myxomycetes. Tree species whose decaying wood yielded myxomycetes included oak, ash, birch, sycamore, silver birch, hazel, .pine and spindle. The only phase of the life cycle isolated directly from within the wood was the uninucleate phase, although aphanoplasmodia did on occasions form on the isolation plates. It was therefore concluded that the uninucleate trophic phase was the most prolonged feeding phase as opposed to the plasmodial phase. Also the predominant type of myxomycete within the decaying wood sampled was the aphanoplasmodial-type. A numerical investigation of the uninucleate myxomycetous cells, flagellates and ciliates yielded by the interior wood of 121 decaying branches, collected from the ground or "aerially" at Leigh Woods, Avon; Wetmoor, Avon; and Weston Big Wood, Avon, revealed numbers of myxomycetous cells up to 64,100 per cm 3 in oak and up to 13,200 per cm 3 in lime. The numbers of uninucleate myxomycetous cells per cm 3 were found to be inversely related to 3 the numbers of non-myxomycetous flagellates and ciliates per cm , possibly reflecting direct competition for bacterial food. The state of decay of the branches was determined by measuring the density and C:N ratio of the wood. These two variables were found to be directly related. No direct relationship between the density of the wood or C:N ratio (as measures of decay) and the numbers of myxomycete uninucleate cells was found. However myxomycetes, non-myxomycetous flagellates and ciliates were more frequently isolated from the lower density (more decayed) wood. A microcosm system representing decaying wood was developed to investigate the position of the myxomycetes in food chains within decaying branches. The uninucleate myxomycetous cells of Ceratiomyxa fruticulosa were found to feed on the bacteria (Aerobacter aerogenes) in the microcosm. There was no evidence that they could directly utilize the cellulose carbon source provided. The fungus (Peniophora guercina) was not noticeably affected by the presence of the other two organisms, although it appeared to have a" deleterious effect on the myxomycete cell numbers. This effect may have been specific to the species of myxomycete and fungus involved. The terminology used to refer to the flagellate and nonflagellate uninucleate trophic cells of myxomycetes is discussed and a new term "myxomonads" is introduced to refer to both the flagellate and non-flagellate cells. The identification of myxoflagellates is also discussed and criteria for their identification listed. The identification of typical myxoflagellates was found to be necessary for their enumeration.