Anyone happen to have optimization hints for this loop?

[dp_pearce]

I have some code that takes data from an Access database and processes
it into text files for another application. At the moment, I am using
a number of loops that are pretty slow. I am not a hugely experienced
python user so I would like to know if I am doing anything
particularly wrong or that can be hugely improved through the use of
another method.

Currently, all of the values that are to be written to file are pulled
from the database and into a list called "domainVa". These values
represent 3D data and need to be written to text files using line
breaks to seperate 'layers'. I am currently looping through the list
and appending a string, which I then write to file. This list can
regularly contain upwards of half a million values...

count = 0
dmntString = ""
for z in range(0, Z):
    for y in range(0, Y):
        for x in range(0, X):
            fraction = domainVa[count]
            dmntString += "  "
            dmntString += fraction
            count = count + 1
        dmntString += "\n"
    dmntString += "\n"
dmntString += "\n***\n

dmntFile     = open(dmntFilename, 'wt')
dmntFile.write(dmntString)
dmntFile.close()

I have found that it is currently taking ~3 seconds to build the
string but ~1 second to write the string to file, which seems wrong (I
would normally guess the CPU/Memory would out perform disc writing
speeds).

Can anyone see a way of speeding this loop up? Perhaps by changing the
data format? Is it wrong to append a string and write once, or should
hold a file open and write at each instance?

Thank you in advance for your time,

Dan