library(dunn.test) #available on CRAN

## count data
df <- data.frame(group=LETTERS[1:3], count = c(1,3,5,4,4,6,1,3,5,2,3,5,1,5,3,4,2,5))

kruskal.test(count~group, data=df)

#Kruskal-Wallis rank sum test
#
#data:  count by group
#Kruskal-Wallis chi-squared = 8.7041, df = 2, p-value = 0.01288






# non-formula usage (default "holm" method)
dunnTest(df$count,df$group)

#Dunn (1964) Kruskal-Wallis multiple comparison
#  p-values adjusted with the Holm method.
#
#  Comparison         Z     P.unadj      P.adj
#1      A - B -1.131517 0.257837400 0.25783740
#2      A - C -2.925386 0.003440288 0.01032086
#3      B - C -1.793869 0.072834082 0.14566816