The thesis explores the dissolution of human subjectivities and creation of new posthuman identities in two trilogies, "MaddAddam" by Margaret Atwood and "Xenogenesis" by Octavia Butler. By creating worlds characterised by nuclear catastrophes, environmental degradation, and abuse of biotechnological progress, the trilogies consider the ways in which the notion of human exceptionalism led to virtual uninhabitability of the planet. While Atwood creates a world of a high concentration of power, overwhelming genetic manipulation of human and nonhuman species, and intense split between nature and culture, Butler depicts alien abduction of humans with a plan to save them from a nuclear disaster and return them to Earth, but under the condition of creating a new hybrid species. Motivated by posthumanism as a theoretical framework, this thesis looks into efforts of maintaining human subjectivity as a closed and fixed system, as well as the transhumanist notion of perfecting the human body, both of which expose the ideological heritage according to which humans are autonomous beings capable of subduing their environment and controling their destiny. On the other hand, the trilogies use the dramatic moment of the suspension of culture as a turning point, a moment potent with the possibilites of adopting new permeable and transforming subjectivities. By utilizing notion such as Deleuze and Guattari’s becoming, Haraway’s cyborg and companion species, and Lynn Margulis’s reconsideration of evolutionary processes, the thesis addresses human-nonhuman and human-alien interactions as a space of multiplicities which reconfigure human subjectivities. Therefore, the trilogies suggest new ways of reorganization of society that would be based on non-hierarchical and sustainable relations and new articulations of human subjectivity driven by its unpreventable fragmentation and hybridization.