- Acer is a large and important genus of woody plants most commonly encountered as small to large trees in urban landscapes. Considerable investigation has been devoted to addressing the taxonomy of maples, but little is known about genome sizes across the genus. Relatively more work has been conducted to determine chromosome numbers and ploidy of more species, but much could be gained by expanding knowledge of genome sizes in combination with traditional cytology. Furthermore, base pair (bp) composition may have implications for a species' adaptability and also impacts nucleic acid stability at high temperatures. Our objectives were to determine the genome size of 195 accessions of maples, assign ploidy to each using inference as well as cytology, and determine base composition of a subset of 48 accessions. Most species had small genome sizes (1.4-3.5 pg) with the exception of section Rubra, which contains many polyploids. Holoploid genome sizes ranged from 1.39 to 6.10 pg, with the latter being interpreted as 9x. The mean monoploid genome sizes (1Cx) ranged from 0.43 pg in A. carpinifolium (section Indivisa) to 1.66 pg in A. caudatifolium (section Macrantha); mean monoploid genome sizes were significantly different among sections. Forty-four of the 48 accessions measured using both fluorochromes had greater estimates with 4',6-diamidino-2-phenylindole (DAPI) than propidium iodide (PI). The proportion of the genome composed of guanosine and cytosine (GC%) among the taxa evaluated in this study ranged from just 38.61% to 43.96% and did not appear to be related to ecological adaptability or urban tolerance among these taxa.