Nanocomposite materials were prepared from an elastomeric medium-chain-length poly(hydroxyalkanoate) (Mcl-PHA) latex as semicrystalline matrix using a colloidal suspension of hydrolyzed cellulose whiskers as natural and biodegradable filler. After stirring, the preparations were cast and evaporated. High-performance materials were obtained from this system, preserving the natural character of PHA. However, differences were reported by comparison with amorphous PHA filled systems. These differences were ascribed to a transcrystallization phenomenon of semicrystalline PHA on cellulose whiskers, evidenced by dynamic mechanical analysis. Transcrystallization hindered the mechanical percolation of cellulose whiskers and the formation of a rigid network within the polymer matrix during the film formation by evaporation. The whiskers network can reorganize under thermal aging.