The insulation for extruded medium voltage power cables currently used world-wide are based on two broad categories of polymeric materials: crosslinkable elastomers, consisting of Ethylene Propylene Rubber (EPR)*, and crosslinkable low density polyethylene, consisting of Crosslinked Polyethylene (XLPE) or Tree Retardant Crosslinked Polyethylene (TRXLPE). These two broad categories have over 40 years of proven field performance. A considerable amount of research has been done on water treeing in unfilled semi-crystalline and amorphous elastomers. Very little research has been done on filled elastomeric systems, as water trees are more difficult to detect. One paper reports a link between ethylene content and crystallinity in an unfilled EPR base resin and other ethylene base resins. In reality, unfilled EPR resin is not a commercially viable insulation material for high demanding medium voltage applications. In filled EPR resin-based insulation materials, surface-treated clays are chemically bonded to the polymer chains. This changes the whole interpretation of degree of crystallinity (related to ethylene content) with respect to bowtie tree formation and long-term wet electrical performance. This paper will report data that shows bowtie tree counts in carefully formulated higher ethylene content filled EPR insulation materials can be less than those for lower ethylene content and more amorphous EPR insulation materials in long-term wet electrical tests. This paper examines the influence of the ethylene content of base resins in suitably reinforced (with treated clay) filled EPR insulation materials with respect to its level of dispersion, formation of bowtie trees and ac voltage breakdown strength after accelerated wet-aged testing. However, the long-term wet ac breakdown strength testing via a step-rise high voltage time test (HVTT) after an accelerated water treeing test (AWTT) are found to be comparable in higher ethylene content (semi-crystalline) and lower ethy- - lene content (more amorphous) filled EPR insulation materials.