NA when using read.csv

Hi, when I used:

Dist=read.csv("test.csv",header=TRUE)

to read data from CSV file. For some cells, R
mistakenly put in as NA, while most of the cells still
appears to be right, and there is no error message in
R. I am pretty sure the csv file is all right, but
just can't figure out what went wrong. Can someone
share your thoughts with me? Thanks!

Ed