Abstract: An insulator for supporting a conductor of an electric fence on a support post in which the insulator is an elongated sleeve formed of a dielectric plastic material. The sleeve has a bore which slidably receives the conductor therein and is formed with a plurality of ribs which extend longitudinally along the conductor and in a radially outwardly extending direction. The ribs are formed integrally with the sleeve and of the same dielectric plastic material. Two of the ribs abut against the fence post to position the sleeve whereby at least one of the ribs extends outwardly from the sleeve. This outer rib is indented by the web portion of an attachment staple, the legs of which are embedded in the post and trap the sleeve therebetween. This indentation of the outer rib prevents the sleeve from sliding along the conductor and out of contact with the post without the use of any fasteners that pierce the insulator sleeve body.