RNA viruses are renowed for their genetic variability. The human immunodeficiency viruses (HIV) are no exception. A rapid method has been established for the genetic identification and differentiation of viral strains based on the sequencing of many M13 clones of gene-amplified products. Some isolates are internally relatively homogeneous while others are heterogeneous. There was no correlation between virus complexity and disease stage. One isolate was in fact a mixture of two distinct strains. A strong preference for G----A base substitutions was observed. These data indicate that HIV isolates cannot be described in simple molecular terms and should rather be considered as quasispecies.