Header for a heat exchanger and resulting heat exchanger are disclosed. The tubular header is of the type which can receive at least one connecting flange (5, 6) with an internal fluid passage (25), and is composed of: a header plate (8) with a longitudinally open wall, provided with slots intended to receive parallel fluid flow tubes; and a cover (9) which, after assembly, closes a longitudinal opening of said header plate, the wall of this cover having at least one external collar (28) delimiting a hole for the flow of fluid (27), said connecting flange (5, 6) being attached to said cover (9) and being configured in such a way that it can be crimped onto the header (3, 4), thereby bringing the internal passage (25) into fluid communication with the hole (27) in said collar. Advantageously, said collar (28) has a wall thickness (33) which is reduced relative to that of said cover (9) from which the collar extends, thereby maximizing the cross section of said hole (27) for the flow of fluid toward the internal passage (25) of said flange.