Pls How can I make 16F877A use 15MHz Xtal

[skimask]

Hi I am using a PIC 16F877A in my project. The Project works fine with a 4MHz crystal but when I replace the 4MHz crystal and its associated capacitors with a 15 MHz crystal and associated capacitors, The pic does not work
Does anyone know what needs to be done in PIC basic to make the 16F877A work?
Thanks

XT vs. HS...
Look in the datasheets.