SUMMARYA new species ofDictyostelium, isolated from leaf mold collected in Poza Rica, Mexico, is described. The slime mold is characterized by the relatively small dimensions of its spores and myxamoebae, and more especially by its diminutive sorocarps—hence the nameD. deminutivum.The slime mold may be cultivated satisfactorily in association with eitherEscherichia coliorPseudomonas fluorescensupon agar media of very low nutrient content. Optimal growth and development occur in association with the latter bacterium upon 0.025% glucose-0.025% peptone agar when glass Petri plate lids are replaced with porous clay covers at the time cell aggregation begins.Aggregations that subsequently produce normal sorocarps typically appear mound-like and arise by the concerted influx of unaligned myxamoebae, either singly or in small groups, rather than by the formation and convergence of definite streams. Depending upon their size, such pseudoplasmodia may occasionally yield single fructifications, but as a rule they produce several sorocarps which arise near the periphery of the cell mass.In the completed sorocarp, the sorophore tapers gradually from base to apex and consists uniformly of a single tier of cells, the outlines of which may be difficult to discern, particularly in the terminal area.