Single fiber strengths of greenhouse-grown Texas Marker 1 (TM 1) and Pima S7 cotton fibers at varying stages of development from 20 days pass anthesis to maturity are reported and compared with those of Maxxa and SJ-2 cotton fibers. The forces required to break single fibers increase most significantly during the fourth week and are similar between TM 1 and Pima varieties through the end of the fourth weeks of development. Beyond 30 dpa, both the single fiber breaking forces and tenacities of the Pima cotton fibers are higher than those of the TM 1 cotton fibers. Variations of single fiber strength from different ovule locations, i.e., chalazal end, medial regions, and micropylar end, as well as along fibers from the medial sections of the ovule are also reported.