PURPOSE: To obtain new hydrophilic fiber, composed of a polymer, having vinyl alcohol units and vinyl ester units with high bulky substituent groups in side chains and exhibiting a structure close to that of multiblock polymers and excellent in (hot) water resistance and strength.

CONSTITUTION: Fiber having structural units expressed by formulas I and 11 (R1 is H or hydrocarbon; R2 and R3 are hydrocarbon or R2 and R3 together form cyclic hydrocarbon or R1 to R3 together form the cyclic hydrocarbon) with 10-90mol% content of the units expressed by formula I, 10-90mol% content of units expressed by formula II and ≤0.6 block character (η). The aforementioned polymer is preferably obtained by subjecting a homopolymer having units expressed by formula II to saponifying reaction in the presence of an alkali catalyst. Furthermore, vinyl pivalate or vinyl versatate is preferred as the monomer capable of providing the units expressed by formula II.