An intI-targeted PCR assay was optimized to evaluate the frequency of partial class 2-like integrases relative to putative, environmental IntI elements in clone libraries generated from 17 samples that included various terrestrial, marine, and deep-sea habitats with different exposures to human influence. We identified 169 unique IntI phylotypes (< or =98% amino acid identity) relative to themselves and with respect to those previously described. Among these, six variants showed an undescribed, extended, IntI-specific additional domain. A connection between human influence and the dominance of IntI-2-like variants was also observed. IntI phylotypes 80 to 99% identical to class 2 integrases comprised approximately 70 to 100% (n = 65 to 87) of the IntI elements detected in samples with a high input of fecal waste, whereas IntI2-like sequences were undetected in undisturbed settings and poorly represented (1 to 10%; n = 40 to 79) in environments with moderate or no recent fecal or anthropogenic impact. Eleven partial IntI2-like sequences lacking the signature ochre 179 codon were found among samples of biosolids and agricultural soil supplemented with swine manure, indicating a wider distribution of potentially functional IntI2 variants than previously reported. To evaluate IntI2 distribution patterns beyond the usual hosts, namely, the Enterobacteriaceae, we coupled PCR assays targeted at intI and 16S rRNA loci to G+C fractionation of total DNA extracted from manured cropland. IntI2-like sequences and 16S rRNA phylotypes related to Firmicutes (Clostridium and Bacillus) and Bacteroidetes (Chitinophaga and Sphingobacterium) dominated a low-G+C fraction ( approximately 40 to 45%), suggesting that these groups could be important IntI2 hosts in manured soil. Moreover, G+G fractionation uncovered an additional set of 36 novel IntI phylotypes (< or =98% amino acid identity) undetected in bulk DNA and revealed the prevalence of potentially functional IntI2 variants in the low-G+C fraction.