Two strains of thermophilic photosynthetic bacteria, designated MD-66T (T = type strain) and YI-9, were isolated from bacterial mats in two separate hot springs in Japan. These new isolates were phenotypically similar to Chloroflexus aurantiacus in some respects. They were thermophilic filamentous photosynthetic bacteria that grew well at 55 degrees C either anaerobically as photoheterotrophs or aerobically as chemoheterotrophs. They exhibited gliding motility, produced bacteriochlorophylls a and c, contained chlorosomes, and required thiamine and folic acid as growth factors. However, isolates MD-66T and YI-9 had the ability to rapidly form mat-like dense aggregates of filaments, an ability which has not been observed in any C. aurantiacus strain. Carbon source utilization tests revealed that unlike C. aurantiacus, the new isolates did not utilize acetate, citrate, ethanol, or glycylglycine. An analysis of the carotenoid components revealed that isolates MD-66T and YI-9 contained mainly gamma-carotene and OH-gamma-carotene glucoside fatty acid esters. These isolates also contained only trace amounts of beta-carotene, which is a major carotenoid component (28.4% of the total carotenoids) in C. aurantiacus. The results of DNA hybridization studies suggested that the new strains were genetically distinct from C. aurantiacus (levels of similarity, 9 to 18%), and 16S rRNA sequence comparisons showed that strain MD-66T was related to C. aurantiacus at a similarity level of 92.8%. On the basis of our data, we propose that a new Chloroflexus species should be created for our new isolates; the name of this new species is Chloroflexus aggregans, and the type strain is strain MD-66 (= DSM 9485).