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The title pretty much says it all, I need to know because I want to install new software that has these two versions, and I need to know which one to install.

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To tell if you are running the 32-bit or 64-bit kernel (which matters for some device drivers), launch System Profiler and click on the "Software" heading in the Contents section.

The line "64-bit Kernel and Extensions" will say "Yes" if you are running the 64-bit kernel and "No" if you are running the 32-bit kernel.

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Go to the Apple Menu and select "About this Mac". If you have a Core Duo processor, you have a 32-bit CPU. Otherwise (Core 2 Duo, Xeon, i3, i5, i7, anything else), you have a 64-bit CPU.

Mac OS X is fairly bitness-agnostic, so either should work. If still in doubt, use the 32-bit version.

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For completeness, there was also the 32-bt only “Core Solo” processor used in the very first Intel Mac mini (“Early 2006”). –  Chris Johnsen Apr 23 '11 at 4:13
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Another option is to use sysctl in a shell:

$ sysctl hw.cpu64bit_capable

It’ll show 1 if the CPU is capable of running 64-bit programs and 0 otherwise.

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The arch command with no arguments will display the machine's architecture type.

The results (from the arch(1) manpage):

i386    32-bit intel
ppc     32-bit powerpc
ppc64   64-bit powerpc
x86_64  64-bit intel
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Uh, no. I have a Core i7 MBP and have the 32-bit kernel enabled since my employer's VPN software requires it, and arch returns i386 for me. My processor is 64-bit and i have lots of 64-bit processes running, though, so this seems to only tell you what the kernel type is. –  David Apr 22 '11 at 23:42
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What David said - this only tells you what kernel is running. Like David, I'm using a 32-bit kernel because I need a kext that requires it, but I can run 64-bit user processes with no problem. –  Sherm Pendley Apr 23 '11 at 1:05
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