Cultivation-independent molecular surveys have shown members of the bacterial phylum Chloroflexi to be ubiquitous in various natural and artificial ecosystems. Among the subphylum-level taxa of the Chloroflexi known to date, the formerly uncultured 'subphylum I' had well been recognized as a typical group that contains a number of environmental gene clones with no culture representatives. In order to reveal their ecophysiology, attempts were made over the past decade to domesticate them into laboratory cultures, and significant advances have been made in cultivating strains belonging to the group. The microorganisms characterized so far include seven species in six genera, i.e., Anaerolinea, Levilinea, Leptolinea, Bellilinea, Longilinea, and Caldilinea, and were proposed to represent two classes, Anaerolineae and Caldilineae, providing solid insights into the phenotypic and genetic properties common to the group. Another subphylum-level uncultured group of the Chloroflexi, i.e., the class Ktedonobacteria, has also been represented recently by a cultured strain. In addition to the results from these tangible cultures, data obtained from functional analyses of uncultured Chloroflexi populations by assessing substrate uptake patterns are accumulating at an encouraging rate. In this review, recent findings on the ecological significance and possible ecophysiological roles of 'Chloroflexi subphylum I' are discussed based on findings from both the characteristics of the cultured Chloroflexi and molecular-based analyses.