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Chapter 1 consists of an introduction to the Myxomycetes (acellular slime-moulds) followed by a description of the culture techniques employed in the study of the organisms. This includes the culture of the haploid amoebae, their crossing to give diploid plasmodia, the culture of plasmodia and the induction of sporulation, and the hatching of spores to give amoebae. Chapter 2 consists of a description of the analysis of fusion behaviour of plasmodia. The behaviour of two plasmodia when they make contact, fusion or non-fusion, was found to be controlled by two genes, four alleles of gene f ( f1 –f4 ) being detected, and two alleles of gene n ( n1 –n2 ). Plasmodia must have the same f genotype for fusion to be possible, and the same n phenotype, ( n2 is dominant to n1 ). One exception to this rule occurs, f3 f3 homozygotes fuse with f4 f4 homozygotes. Based partly on deductions from this exceptional observation, a general model for the mode of action of the f and n genes is proposed. The analysis of fusion type was complicated by the occurence of a recessive allele, qˉ, linked to the f gene. In the homozygous state the qˉ allele results in restricted nutritional behaviour, death occurring on the usual axenic culture medium. This is the first report of linkage in Physarum polycephalum. The genes detected during the analysis of fusion behaviour were made use of in the analysis of senescence. Chapter 3 reports the further analysis of an actidione resistant mutant which, it was hoped, would be of use as a marker in the study of senescence. Chapter 4 reports the application of genetic markers to the study of the ploidy of plasmodia. Chapter 5 begins with an introduction to the phenomenon of senescence. This is followed by the first report of senescence in a Myxomycete. Three main techniques were applied to the analysis of this phenomenon; a) The life-expectancy of heterokaryons formed by fusing equal quantities of plasmodia of known, and disparate, life-expectancy was studied. b) The f gene was used to study the stability of the nuclear ratios of such heterokaryons. c) Various procedures, for example mutagenesis, were applied to plasmodia as a pulse, and the life-expectancy of the treated plasmodia observed to see whether senescence had been affected by the procedure. The data produced by these three techniques suggests the hypothesis that senescence is due to the accumulation of defective mitochondria.