An extensive study was made on the moss flora of the Bío-Bío Region (VIII Región) in south-central Chile in 2001-2003. Collections were made in all four provinces of the region: Arauco, Bío-Bío, Concepción and Ñuble. Approximately 265 localities in the region were explored with over 6,000 mosses collected in the four provinces. The mosses of this region had not previously been studied to any great extent and with part of the region’s environment being destroyed by the construction of several dams on one of the major rivers, the Bío-Bío, the study of this area seemed of utmost importance. Thus far, a total of 20 taxa were found which are new to Chile, making a total of 877 known for the country, with four new taxa known for South America. An additional 87 taxa are reported new only to the Bío-Bío Region. That number, together with some new records from the recent literature, increases the total for the Region from 190 to 300. It was determined from the 87 new taxa for the Bío-Bío Region that the majority (41) represent northern extensions of taxa, while a much smaller number (10) represent southern extensions. The remainder (36) fill in a gap in the distribution of the taxa between the northern and southern parts of the country. Many difficult species still remain to be identified and the number of species new to science, to Chile and to the Bío-Bío Region, is certain to increase when the remaining specimens are identified.