Unfixed tissue from the human ovary, the tail tendon and skin from the tail of rats are incubated in 5% solutions of uranyl acetate. The collagen fibrils of all tissues are decomposed and display an obvious tridimensional spiral structure on all levels. The fibril is a complex biopolymer constructed out of filaments, surrounded by and associated with an amorphous cementing matrix. The filaments consist of 3 to 5 subfilaments with a thickness of 30--45 A spirally wound around one another. The filaments are twisted along the length of the axis of the fibril under a definite angle of inclination and a pitch of the spiral equal to 1.04--1.12 micrometer for the ovary and from 2.6 to over 5.6 micrometer for the tail tendon. The cross striations seem to spring out of the nodular thickenings along the filaments. Bridge-like connections corresponding to the separate striations are established between adjacent fibrils. A new tridimensional structure model of the collagen fibrils is proposed.