problem with read.table

Hi All

recently i faced an unknown problem while reading the data. Can
someone help me in understanding why this happened.

I have .txt file containing X, Y, Z variables. I used the command

a <- read.table("filename", header=TRUE)

after reading the file i am able to view it by tryping a. but i am
unable to access the variables in a, by giving the command a$X.

a$X
NULL

this is the output it was showing. However the same i am able to
access by giving a[[1]]. i tried changing the mode by issuing command
a <- as.data.frame(a). But situation remains the same.

look forward for experts suggestion on this.

rgds

snvk