We have analysed the ability to infect pigs of two foot-and-mouth disease virus (FMDV) variants isolated at low frequencies from virus populations (quasispecies) generated in pigs on infection with a parental virus, C-S8c1. A monoclonal antibody-resistant mutant (MARM21), and a variant isolated at early times post-infection (S-3T1), each exhibiting a unique amino acid substitution in VP1, were able to cause disease in pigs, both by direct inoculation or by contact transmission. The symptoms developed were similar to those produced by C-S8c1 or the related virus C-S15c1. The VP1 sequence of viral RNA directly recovered from lesions of infected animals confirmed the stability of the variant genotypes. Pigs infected with S-3T1 consistently showed an advance of 12 to 24 h in the emergence of fever and lesions when compared to animals infected with C-S8c1 or the remaining variants, an observation consistent with its early isolation. The ability of FMDV variants to compete in vivo with C-S8c1 was investigated in co-infection experiments. Analysis of the proportion of each of the competitors in lesions of co-infected pigs revealed that none of the variants was completely overgrown by the parent. However, co-infection with C-S8c1 and MARM21 resulted in lesions in which C-S8c1 was predominant, indicating a selective disadvantage of this variant in swine. In contrast, lesions from swine co-infected with C-S8c1 and S-3T1 contained similar proportions of the two viruses. These results document fitness variations in vivo among components of the mutant spectrum of FMDV quasispecies.