A helically grooved flexible optical cable core comprising a plurality of helically laid flexible bodies 1 of sector-shaped cross-section is manufactured by drawing a multiplicity of flexible non-metallic reinforcing elements 2 under tension through electrically insulating plastics material 3 in a semi-liquid state; causing the plastics encapsulated reinforcing elements to pass through an elongate die 23 having an outlet orifice 25 of sector-shape, which die, between its ends, is of such a shape that reinforcing elements adjacent the arcuate surface 4 of the body are travelling at a greater linear speed than reinforcing elements remote therefrom, and which has a blade 26 protruding inwardly of the die and forming in the arcuate surface of the body a longitudinally extending groove 5; causing the body as it emerges from the die to follow a helical path with its grooved arcuate surface outermost and so cooling the helically advancing body that the plastics material sets; partially filling the groove in the arcuate surface with a grease and continuously feeding an optical fibre into the grease in the groove; laying up the helically advancing body with a plurality of helically advancing bodies of similar cross-section and construction to form a flexible core of circular cross-section which is rotating about its longitudinal axis; and taking up the rotating core in such a way that any residual torsion in the core arising from said rotation is removed.