We determined the temporal and seasonal dynamics of intra-annual cell formation of south Florida slash pine (Pinus elliottii Engelm. var. densa Little & Dor.), the southernmost native pine in the United States and the foundation species of globally endangered pine rockland ecosystems. To assess intra-annual cambial activity and identify possible relationships between cell production and climatic factors, wood micro-cores were extracted monthly from six trees during the period March 2010 to March 2011. The results confirmed annual growth ring formation in P. elliottii var. densa and indicated that its growing season extends from February to December, with a short period of dormancy that varied little between individuals. Within the growing season, earlywood cells were produced from February to July, latewood cells produced from July to December, and intra-annual density fluctuations (IADFs) occurred in the growth rings of four of six trees between July and August. A principal component analysis indicated a homogeneous response of cambial activity among trees to site-specific climatic factors. The first principal component axis explained 71 % of the total variance in cell production during the study period. Our results indicated that the dynamics of seasonal cambial activity of P. elliottii var. densa are controlled by solar radiation (r = 0.51, p < 0.10) in the Florida Keys. The nature of our data allow us to only speculate on the ecophysiological processes responsible for IADFs in P. elliottii var. densa, and additional research is needed to better understand the relationship between their formation and the environment in the Lower Florida Keys.