how make category archive posts in reverse-chronological order

Do not edit the Twenty Eleven theme. It is the default WordPress 3.2 theme and having access to an unedited version of the theme is vital when dealing with a range of site issues.

Create a child theme for your customisations. Once your child theme is active, see Category_Templates and query_posts.

Create a child theme for twenty ten, then copy across loop.php to the child theme, rename the copied file loop-category.php

Open the file and before these lines:

<?php /* If there are no posts to display, such as an empty archive page */ ?>
<?php if ( ! have_posts() ) : ?>

Add a line to reverse the sort order!
<?php query_posts($query_string . "&order=ASC"); ?>

So you have:

<?php /* Reverse the sort order ascending */ ?>
<?php query_posts($query_string . "&order=ASC"); ?>

<?php /* If there are no posts to display, such as an empty archive page */ ?>
<?php if ( ! have_posts() ) : ?>

Twenty Ten Child Theme:
If you want to make it better and category specific, or do not want to code it yourself, I have already posted about this and there is a child theme, I use it for a series of posts like, part 1, part 2 etc:

So if you have no other changes, job done!

Twenty Eleven Child Theme (Page of Posts):
This variation uses Custom Fields, and is a template page rather than index.pgp or category.php, and is for twenty eleven.

HTH

David

Thank you both so much!
I will try that.

thanks.

actually, when I do that, the sort order is reversed successfully, however any category archive page shows ALL posts.
Was I also supposed to copy across anything else other than style.css and loop.php to the child theme?

thanks!