I've had Photoshop 7 since it first came out, but have only dabbled lightly in it a couple of times.

I am trying to make a background picture to use in Poser (I have have both 5 and 7) of a subsurface view of an ant colony with an abouve ground view too.  I created the scene using two layers with heavy noise filter for the subsurface ground texture and ligt noise for the background of the tunnles, then erased the aboveground view on the layer with the light noise.  I'm left with a .psd file as a background in Poser in which I need to make the above ground portion transparent in Photoshop so I can add aboveground elements elements (plants, peiple, etc.) in Poser and not have them show against the white aboveground background.  I can use the magnetic lasso tool to select the white aboveground view portion in Photoshop, but I'm stuck knowing what to do next to make this portion transparent in the final .psd file for imprtinging into Poser.

If I understand you right, then you can just flatten the image, select the part you don't want by whatever means, then delete the selection. The deleted section will be transparent and you can save it as a .psd or .png.

I think another problem may be that Poser doesn't appear to honour transparency either in the background image or when applied to an object. A workaround might be to create a transmap and instead of putting your background image into Poser's background, create an appopriately sized and positioned plane and apply the image to that, so the plane acts as a background. Then you can apply your transmap to the plane and mask off the part you don't want to to see.

When creating new document in PS, check "Transparent" box on very bottom. It will create transparent document. Or DONT work on background layer and delete it before saving. Dont flatten the layers but merge them to preserve transparency.

You can save transparent image as .TIFF or .PNG (.GIF is for web).

Tihomir