# Some mock data
set.seed(2014)
Y<-rpois(1000,4)
set.seed(2014)
x<-2*Y+0.1*rnorm(1000)

# Estimate model
require(MASS)
m1<-glm(Y~x,family="poisson"(link=log))

# Get Pseudo R²
require(pscl)
pR2(m1)

         llh          llhNull          G2      McFadden          r2ML          r2CU
-1634.4075357 -2080.4918645   892.1686575     0.2144129     0.5902339     0.5995827

# Estimate model with intercept only
m0<-glm(Y~1,family="poisson"(link=log))
R<-1-logLik(m1)/logLik(m0);R

'log Lik.' 0.2144129 (df=2)