Abstract: In a slotted core optical cable in which each slot contains stacked optical fiber ribbons, the cost of the cable can be reduced and cooling of the slotted core during manufacture of the cable is facilitated as compared with an optical cable in which the slots of the slotted core are all of the same depth by forming the circumferentially spaced slots into two groups, the slots of each group being of the same depth as one another and the depth of each of the slots of one group being greater than the depth of each of the slots of the other group, and by arranging for each slot of one group to be disposed between two slots of the other group. The number of optical fiber ribbons 5 stacked in each of the deeper slots is greater than the number of optical fiber ribbons stacked in each of the other slots. Approximately the same number of optical fiber ribbons can be accommodated in slots of a core of smaller diameter than hitherto achieved.