Cell competition involves a conserved fitness-sensing process during which 
fitter cells eliminate neighbouring less-fit but viable cells1. Cell competition 
has been proposed as a surveillance mechanism to ensure normal development and 
tissue homeostasis, and has also been suggested to act as a barrier to 
interspecies chimerism2. However, cell competition has not been studied in an 
interspecies context during early development owing to the lack of an in vitro 
model. Here we developed an interspecies pluripotent stem cell (PSC) co-culture 
strategy and uncovered a previously unknown mode of cell competition between 
species. Interspecies competition between PSCs occurred in primed but not naive 
pluripotent cells, and between evolutionarily distant species. By comparative 
transcriptome analysis, we found that genes related to the NF-κB signalling 
pathway, among others, were upregulated in less-fit 'loser' human cells. Genetic 
inactivation of a core component (P65, also known as RELA) and an upstream 
regulator (MYD88) of the NF-κB complex in human cells could overcome the 
competition between human and mouse PSCs, thereby improving the survival and 
chimerism of human cells in early mouse embryos. These insights into cell 
competition pave the way for the study of evolutionarily conserved mechanisms 
that underlie competitive cell interactions during early mammalian development. 
Suppression of interspecies PSC competition may facilitate the generation of 
human tissues in animals.