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

Its not likely that there are errors in this software so
its probably not a mistake but some other cause.

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!

Read the posting guide at  the bottom of each post.
In particular, cut down the input file to a few lines that
still exhibit the problem in order to provide a reproducible 
example. 

As an aside note that 'header = TRUE' is the default in read.csv
(but not on read.table) so you don't need to specify it.