We analyze variations in the dendritic fields of spinal motor neurons (sMN) projecting to muscles of the sexually dimorphic anal fin in adult female and male Western Mosquitofish, Gambusia affinis affinis, sMN were labeled using filter paper strips saturated with 3000 MW amino dextran, Texas Red® (TDA) dye. Strips were inserted into rhizotomized intercostal nerves within the spinal cord and allowed to transport 8 hrs. Each spinal cord segment, defined by the presence of a ventral root, is roughly 100 um long, 60 um in diameter, and corresponds in length to a myotomal segment. Retrograde diffusion of TDA revealed the entire soma, dendritic branching points, patterning, and elongation of sMN of female and male G. a. affinis, sMN projections extend beyond their soma position in the myotomal segment, probably to increase synaptic connectivity across multiples segments. In terms of branching and lengthening, the dendritic field pattern is more complex in males than it is in females. Results show that in males sMN radically reorganize their dendritic structure and organization; such reorganization is likely related to the novel teleost function of internal fertilization. Because dendritic field patterns of the sMN in G. a. affinis do not follow described patterns known for spinal motor neurons, reexamination of the characteristics of dendritic aborization patterns is required. [ABSTRACT FROM AUTHOR]