Does damage prevention apply before or after deathtouch is considered?

Damage prevention is applied as damage is being dealt, during the damage-dealing process itself (CR 615.4). Prevention effects replace or reduce the damage before it is ever considered 'dealt.' Deathtouch, by contrast, only functions once damage is actually dealt to a creature (CR 702.2b).

Because prevention comes first in the sequence, if all damage from a deathtouch creature is prevented, zero damage is actually dealt. Since no damage was dealt, deathtouch has no damage to 'count,' and the struck creature will not be destroyed by deathtouch alone (CR 704.5g only checks for lethal damage, and prevented damage is not lethal damage).

This interaction is especially relevant with protection. A creature with protection from black, for example, has all damage from black sources prevented (CR 702.16b). If a black deathtouch creature deals damage to it, that damage is entirely prevented, and the protected creature survives — deathtouch does nothing.

Concrete example: A 1/1 with deathtouch attacks. Your opponent blocks with a creature that has a prevention shield (e.g., from a spell like 'Prevent the next 1 damage dealt to target creature'). The 1 damage is prevented before it is 'dealt,' so the blocking creature takes 0 damage and deathtouch never applies — the blocker survives.

In short: no prevention, no damage; no damage, no deathtouch trigger. Always resolve prevention effects first, then check whether any damage actually got through for deathtouch to act on.