Do you mean .fas file as FASTA file format? If so rename it to .fa or .fasta and SplitTree should read it (should work in SplitsTree4 version). Let me know whether this worked.
Thanks for your answer. I have an alignment done in Bioedit and saved as *.fas file, unfortunatelly I am not able to save is as *.fa or *.fasta (it seems it is not possible in Bioedit?). However, I opened my file via Clustar Omega (on-line version) and downloaded the final file *clustal. This can be open in SplitTree (uffff). Thanks once again for your comment,