I have a big problem,my gfortran can not execute

[Timlu]

Hello,
Can you help me ?I have a big problem,my g77,gcc,gfortran can not execute.I use Mac OS X 10.7.3 Lion system,I already install Xcode 4.3,g77,gcc,and gfortran,when I try to execute the .f file,it always appears
[ error trying to exec 'as' : execvp : No such file or directory ] ,but I really sure I have the .f file.
When I open terminal and write 
which gfortran
It will appear
/usr/bin/gfortran
and
which gcc
It also can appear
/usr/local/bin/gcc
But when I write
which as
It will appear nothing
I still do not know why I do not have the 'as' file,and the error ( trying to exec 'as' : execvp : No such file or directory )
is because I do not have 'as' file ?
How can I solve this problem ? I am a newer of Mac
Thanks for your help~
My friends told me,you are so nice,and can help me to solve the problem,so I write the letter to ask you for help !

Best,
Tim Lu