In a negative dobby for forming the warp shed in weaving looms, the stresses on some of the machine components are greatly reduced, whereby a more precise control of the heald frames is obtained, thanks to the fact that the controls transmitted to the crooks (9, 10), engaging the hooks (7, 8), by the means (12) reading the pattern of the fabric to be woven, are no longer affected by the strains deriving from the stresses imparted on said crooks (9, 10). This is obtained by operating the crooks (9, 10) by means of two-armed levers (13), controlled by the spiked roller (12) forming the pattern reading means -and connected by spring means (17) to a square arm (18) of said crooks (9, 10), the rocking movements of said crooks (9, 10) and of said two-armed levers (13) being controlled by two distinct shafts - (9A, 14).