In Mytilus mussels, paternal mitochondrial DNA (M type) from sperm is known to be transmitted to offspring. This phenomenon is called doubly uniparental inheritance (DUI). Under DUI, it has been reported that female mussels generally have only maternal mtDNA (F type). In this study, we examined the mode of mtDNA transmission in Mytilus galloprovincialis using M and F type-specific primer sets. The ratio of M and F types were measured in each sample by SNaPshot. The M type was detected in the adductor muscle and female gonad of all females. In unfertilized eggs spawned by 84.6% of females (22/26), M type was also detected. The F type was more abundant than the M type in all females. Although the ratio of M type in females was very low, all females contained the M type. From these results, we propose a new possibility about DUI inheritance. The presence of M type in unfertilized eggs indicates that the M type of eggs may also contribute to M type inheritance.