The troglobitic harvestman Jimeneziella decui Avram, 1970 is known from four neighboring caves (Cueva de Majana, Cueva de los Golondrinos, Cueva Perla del Agua, and Cueva de Máximo) located in eastern Cuba. We present the first ecological data on a population of this endangered species in Cueva de Máximo. The sex ratio of the population estimated in the main gallery of the cave was not different from 1:1. The spatial distribution observed was uniform, and the density of individuals was 0.48 individuals/m² (February–March) and 0.84 individuals/m² (November). We describe morphological differences between the sexes and between males. Preliminary morphological and behavioral data suggests the possible existence of two male morphs in J. decui (“robust” and “slender” males). The slender males possessed less developed armature on leg IV; and the chelicerae, coxa IV and femur IV were less swollen than robust males. Our observations on male-male interactions suggest that robust males are more aggressive than slender males, which never initiated an attack on robust males, but in some occasions responded to attacks from robust males. The non-aggressive behavior exhibited by robust males towards slender males also suggests that the slender morph is not only a sneaker, but potentially a female mimic, which is also consistent with the morphology of slender males. Ecological and behavioral information also suggests the possible existence of territories defended by robust males with their well-developed weapons, and the presence of females inside them.