<p>I need to programatically determine whether .NET 3.5 is installed. I thought it would be easy:</p>

<pre><code>&lt;% Response.Write(Environment.Version.ToString()); %&gt;
</code></pre>

<p>Which returns "2.0.50727.1434" so no such luck...</p>

<p>In my research I have that there are some rather obscure registry keys I can look at but I'm not sure if that is the route to go. Does anyone have any suggestions?</p>

That is because technically .NET 3.5 is an extension of the 2.0 framework.  The quickest way is to include an assembly from .NET 3.5 and see if it breaks.
```
```
System.Web.Extensions
```
```
Is a good assembly that is only included in version 3.5.  Also it seems that you are using ASP.NET to run this check, this really limits you because you will be unable to check the file system or the registry running in the protected mode of ASP.NET.  Or you can always problematically try loading an assembly from the GAC that should only be in .NET 3.5, however you may run in to problems with permissions again.
This may be one of those times where you ask your self "What am I trying to accomplish?" and see if there are alternative routes.